VoLTE Training 2026: Complete Guide to Voice over LTE Networks, IMS & Call Flows

Introduction VoLTE Training 2026

VoLTE Training 2026 Voice calls have come a long way. From crackly analog calls of the 1980s to today's crystal-clear HD voice conversations — the technology underneath has changed completely. At the heart of modern voice communication sits VoLTE, or Voice over LTE. If you work in telecom or plan to, understanding how VoLTE works isn't optional anymore — it's foundational.

VoLTE training has become one of the most sought-after skill sets in the telecom industry as carriers worldwide migrate subscribers from legacy CS (Circuit Switched) networks to IP-based voice delivery. As we move through 2026, operators are not just deploying VoLTE — they're evolving it, integrating it with 5G voice architectures, and using it as the backbone for services like ViLTE (Video over LTE), RCS (Rich Communication Services), and emergency calling over LTE.

Whether you're an aspiring protocol engineer, a network operations professional, or a fresh graduate targeting the telecom industry, this guide covers everything — from IMS architecture and SIP call flows to career opportunities and the best training resources available VoLTE Training 2026.

VoLTE Training 2026

Table of Contents

1. What Is VoLTE? A Clear Technical Overview

2. Why VoLTE Replaced Circuit Switched Voice

3. IMS Architecture: The Engine Behind VoLTE

4. Key IMS Network Entities Explained

5. SIP Protocol: The Language of VoLTE

6. VoLTE Call Flows: Registration and Session Setup

7. VoLTE Call Flows: Voice Codec and Bearer Setup

8. QoS in VoLTE: Dedicated Bearers and QCI

9. VoLTE Handover: SRVCC Explained

10. ViLTE and RCS: VoLTE Extensions

11. VoLTE in 5G Networks: VoNR and EPS Fallback

12. Real-World VoLTE Deployment Examples

13. Troubleshooting VoLTE: Common Issues and Tools

14. Telecom Industry Career Opportunities in 2026

15. Why Apeksha Telecom and Bikas Kumar Singh Are Important

16. FAQs

17. Conclusion

    
    

1. What Is VoLTE? A Clear Technical Overview

VoLTE stands for Voice over LTE. It is the standard mechanism for delivering voice calls over a 4G LTE network using IP-based protocols, specifically the IMS (IP Multimedia Subsystem) framework and the SIP (Session Initiation Protocol).

Before VoLTE, most 4G smartphones handled voice calls by falling back to 3G or 2G circuits — a technique called CSFB (Circuit Switched FallBack). This caused call setup delays, temporary data interruption, and inferior audio quality. VoLTE eliminates CSFB entirely. Voice is transmitted as IP packets over dedicated LTE bearers, giving users simultaneous high-quality voice and data without any speed degradation VoLTE Training 2026.

The ITU and 3GPP standardized VoLTE under the GSMA IR.92 specification, which defines the minimum set of features operators must implement for VoLTE interoperability. This standardization means a VoLTE call from one carrier can seamlessly connect to a VoLTE subscriber on another carrier — even across international boundaries.

Key VoLTE characteristics:

• HD voice using the AMR-WB codec for dramatically better audio clarity

• Faster call setup — typically under 3 seconds compared to 6–8 seconds with CSFB

• Simultaneous voice + data at full LTE speeds — no fallback, no speed reduction

• Lower battery consumption compared to CSFB

• Video calling support (ViLTE) built on the same IMS infrastructure

By 2026, VoLTE has become the default voice delivery mechanism for virtually every major LTE operator globally.

2. Why VoLTE Replaced Circuit Switched Voice

In 2G and 3G networks, voice was handled using circuit switching. A dedicated circuit was reserved between caller and receiver for the duration of the call — reliable but inefficient.

When LTE was designed, the architects made a deliberate choice: LTE would be a pure packet data network with no circuit switching. This was the right long-term decision, but it created an immediate problem — how do you carry voice on a packet data network with the quality, reliability, and latency that real-time voice demands?

The answer was IMS — originally defined by 3GPP in Release 5 (2002) and refined through subsequent releases to become the voice and multimedia delivery platform for LTE and beyond.

VoLTE over IMS gave operators significant advantages:

• All-IP architecture — one unified network for voice and data, lower OPEX

• Rich services — video calling, RCS messaging, conferencing on the same platform

• Better spectrum efficiency — voice as IP packets uses LTE resources more efficiently

• Faster innovation — new voice services deployed as software, not hardware changes

• 5G readiness — IMS is also the voice backbone for 5G (VoNR)

By 2026, virtually every major operator has fully deployed VoLTE and is actively sunsetting 3G networks whose primary remaining use case was voice fallback.

3. IMS Architecture: The Engine Behind VoLTE

IMS (IP Multimedia Subsystem) is the core framework that makes VoLTE possible. It's a layered, service-oriented architecture built on top of IP that handles session management, authentication, routing, and service delivery for real-time multimedia services.

The IMS architecture is described in three layers:

Transport Layer (Layer 1)

The LTE/EPC network itself — UE, eNodeB, MME, S-GW, and P-GW. The P-GW connects the LTE network to the IMS core.

Control Layer (Layer 2)

Where IMS lives. Contains all signaling entities: P-CSCF, I-CSCF, S-CSCF, HSS, and PCRF. These handle session control, authentication, and routing.

Application Layer (Layer 3)

Contains Application Servers (AS) that deliver specific services — voicemail, call waiting, RCS messaging, conferencing. AS entities communicate with the S-CSCF via the ISC interface.

Understanding this three-layer model is the starting point for any serious VoLTE training curriculum.

4. Key IMS Network Entities Explained

P-CSCF (Proxy Call Session Control Function)

The P-CSCF is the UE's first point of contact in IMS. It acts as a SIP proxy for all messages from the UE, enforces QoS for VoLTE bearers by interacting with the PCRF, and is typically operated by the visited network.

I-CSCF (Interrogating CSCF)

The I-CSCF is the entry point to a home network. It queries the HSS to find which S-CSCF a subscriber should use, and can hide internal IMS topology from external networks (THIG function).

S-CSCF (Serving CSCF)

The central signaling node in IMS. Handles SIP session registration, routing, and service trigger logic. Authenticates subscribers using AKA and triggers Application Servers based on iFC (initial Filter Criteria) from the HSS.

HSS (Home Subscriber Server)

The master subscriber database. Stores subscriber profiles, authentication vectors, S-CSCF assignment, and service profiles. Communicates with CSCFs via Cx and Dx interfaces using Diameter protocol.

PCRF (Policy and Charging Rules Function)

Responsible for policy control and QoS enforcement. When a VoLTE call is established, the P-CSCF notifies the PCRF, which authorizes and provisions a dedicated EPS bearer with the appropriate QCI for voice traffic.

MGCF and IMS-MGW

Handle interworking between IMS (VoLTE) and legacy PSTN networks, converting between SIP signaling and ISUP and between IP voice and TDM circuits.

5. SIP Protocol: The Language of VoLTE

SIP — Session Initiation Protocol — defined in RFC 3261, is the application-layer protocol VoLTE uses for signaling. It handles session setup, modification, and teardown. SIP is text-based, modeled after HTTP, carrying SDP (Session Description Protocol) in its body to describe media parameters.

Key SIP methods used in VoLTE:

• REGISTER — UE registers with IMS S-CSCF, binding its public identity to its current IP address

• INVITE — initiates a VoLTE call; contains SDP offer with proposed media parameters

• 100 Trying — provisional response indicating the request is processing

• 180 Ringing — called party's phone is ringing

• 200 OK — call accepted; contains SDP answer with agreed media parameters

• ACK — completes the three-way handshake (INVITE → 200 OK → ACK)

• BYE — terminates an established session

• CANCEL — cancels a pending INVITE before it's answered

• UPDATE — modifies session parameters without re-establishing the dialog

• PRACK — provisional response acknowledgment (required in VoLTE for reliable provisional responses)

In 2026, protocol testing engineers spend significant time analyzing SIP message captures using Wireshark, Spirent, and specialized IMS test platforms.

6. VoLTE Call Flows: Registration and Session Setup

Understanding call flows is the core practical skill in VoLTE training. Before a subscriber can make or receive calls, their UE must register with the IMS.

IMS Registration Flow

1. UE powers on and attaches to LTE. MME creates a default EPS bearer. P-GW assigns an IP address and creates an IMS PDN connection.

2. UE sends SIP REGISTER to P-CSCF containing the IMPU (public user identity) and IMPI (private identity).

3. P-CSCF forwards the REGISTER to I-CSCF.

4. I-CSCF queries HSS (Cx UAR) to determine which S-CSCF to assign.

5. HSS responds with the S-CSCF address (Cx UAA).

6. I-CSCF forwards REGISTER to S-CSCF.

7. S-CSCF queries HSS for authentication vectors (Cx MAR/MAA).

8. S-CSCF sends 401 Unauthorized with authentication challenge (RAND and AUTN).

9. UE authenticates the network (mutual authentication via AKA) and sends new REGISTER with computed RES.

10. S-CSCF validates RES, queries HSS to download the subscriber profile (SAR/SAA), sends 200 OK — registration complete.

This flow involves Diameter signaling on the Cx interface and SIP on the Gm, Mw, and ISC interfaces.

7. VoLTE Call Flows: Voice Codec and Bearer Setup

Mobile Originating (MO) Call Setup

1. Calling UE sends SIP INVITE to P-CSCF. Contains SDP offer with proposed codec (AMR-WB for HD voice) and media IP/port.

2. P-CSCF sends AAR (AA-Request) to PCRF to authorize QoS for a voice bearer.

3. PCRF authorizes the bearer and responds with AAA (AA-Answer).

4. INVITE traverses P-CSCF → S-CSCF → I-CSCF → terminating network.

5. Terminating S-CSCF triggers iFC, routes to called party's P-CSCF.

6. Called UE rings — 180 Ringing sent back to calling UE via SIP path.

7. Called party answers — 200 OK sent with SDP answer (agreed codec and media parameters).

8. Calling UE sends ACK — three-way handshake complete.

Dedicated Bearer Setup (QoS)

In parallel with SIP signaling, the LTE network establishes a dedicated EPS bearer for voice traffic:

• QCI 1 — QoS class for conversational voice (lowest delay budget among GBR bearers)

• GBR (Guaranteed Bit Rate) — ensures minimum bandwidth for the voice stream

• ARP (Allocation and Retention Priority) — voice bearers prioritized and protected

Once established, RTP (Real-Time Protocol) packets carry actual voice data between endpoints.

8. QoS in VoLTE: Dedicated Bearers and QCI

Quality of Service is central to VoLTE performance. Without proper QoS, voice packets compete with web browsing and streaming on the same LTE channel.

EPS Bearer Types

• Default Bearer — always-on, non-GBR, used for IMS signaling (QCI 5) and general data

• Dedicated Bearer — established for the voice RTP stream (QCI 1)

QCI Values for VoLTE

QCI	Traffic Type	Priority	Delay Budget	Loss Rate
1	Conversational Voice RTP	2	100ms	10^-2
5	IMS Signaling	1	100ms	10^-6
8/9	Default Data	8/9	300ms	10^-6

The QCI 1 bearer is a GBR bearer, guaranteeing around 25.5 kbps for AMR-WB at 12.65 kbps codec mode.

RTCP and RTCP-XR

RTCP (RTP Control Protocol) accompanies RTP for quality monitoring. RTCP-XR provides detailed metrics including MOS (Mean Opinion Score) estimation, jitter, packet loss, and delay — critical for network operations teams monitoring VoLTE quality.

9. VoLTE Handover: SRVCC Explained

SRVCC — Single Radio Voice Call Continuity — handles the scenario where a VoLTE subscriber moves from LTE coverage into a 3G or 2G area mid-call. It seamlessly transfers the active VoLTE call to a circuit-switched network.

The SRVCC process:

1. eNodeB detects weakening LTE signal and triggers measurement reports

2. MME initiates SRVCC handover procedure

3. Signaling exchanged between LTE MME and 3G MSC (via Sv interface)

4. IMS anchors the call at the MSC Server

5. UE transitions to 3G and resumes using circuit-switched voice

6. SIP session in IMS updated to reflect the new circuit-switched anchor

SRVCC has become less critical in 2026 as 3G networks shut down globally and LTE coverage reaches near-ubiquity. However, it remains on the curriculum because some operators maintain 3G fallback in rural areas.

10. ViLTE and RCS: VoLTE Extensions

ViLTE (Video over LTE)

ViLTE uses the same IMS infrastructure as VoLTE but adds a video media stream. The SIP INVITE includes SDP with both audio (AMR-WB) and video (H.264 or H.265) codecs. Dedicated bearers are established for both streams (QCI 1 for audio, QCI 2 for video). ViLTE enables native video calling directly from the phone dialer.

RCS (Rich Communication Services)

Often called the "SMS replacement," RCS is an IMS-based messaging protocol standardized by GSMA as the Universal Profile. RCS adds:

• Chat and group chat with read receipts and typing indicators

• High-resolution photo and video sharing

• Location sharing

• Business messaging (RCS Business Messaging / RBM)

• File transfer

By 2026, RCS has achieved widespread deployment. Apple adopted RCS in iOS 17, making it the universal messaging baseline. RCS Business Messaging has become a major revenue channel for operators and enterprises.

11. VoLTE in 5G Networks: VoNR and EPS Fallback

EPS Fallback

In early 5G NSA deployments, 5G NR was used only for data. When a subscriber needed to make a voice call, the network fell back to LTE where VoLTE was already deployed — this is EPS Fallback. It also applies to early 5G SA deployments before VoNR is ready.

VoNR (Voice over New Radio)

VoNR is the native voice solution for 5G SA networks — the VoLTE equivalent for 5G. It uses the same IMS infrastructure and SIP-based call flows as VoLTE, but runs over the 5G NR air interface and 5G Core.

VoNR advantages:

• Lower call setup latency than VoLTE

• Support for EVS (Enhanced Voice Services) codec — better quality than AMR-WB

• Better capacity for high-density environments

• Native integration with 5G SA network slicing

In 2026, VoNR deployment is accelerating. Operators in South Korea, China, and parts of Europe have commercially launched VoNR. Understanding VoLTE deeply is the essential prerequisite — the architectures are closely related and the IMS core is shared.

12. Real-World VoLTE Deployment Examples

Reliance Jio (India)

Jio launched as a VoLTE-only operator in 2016 — skipping 2G/3G entirely. By 2026, Jio serves hundreds of millions of VoLTE subscribers, offering VoLTE, ViLTE, and RCS services at massive scale. Jio's deployment is frequently cited in VoLTE training programs as a landmark greenfield deployment.

T-Mobile US

T-Mobile completed its 3G shutdown in 2022, making VoLTE the primary voice technology for its entire subscriber base. T-Mobile's VoLTE deployment also serves as the voice backbone for its 5G SA network, with EPS Fallback transitioning subscribers to VoLTE where VoNR is not yet available.

Deutsche Telekom (Europe)

Deutsche Telekom has deployed VoLTE across Germany, Austria, and Eastern European markets. DT is among the leaders in VoNR trials in 2026, building on its mature VoLTE IMS infrastructure.

13. Troubleshooting VoLTE: Common Issues and Tools

Common VoLTE Issues

• Registration failures — P-CSCF discovery failures, DNS issues, or AKA authentication problems. Requires analysis of SIP REGISTER/401/200 OK flows.

• Call setup failures — SDP negotiation mismatches, PCRF policy issues preventing dedicated bearer creation, or routing problems in IMS core.

• One-way audio — NAT traversal problems, ICE failure, or incorrect SDP IP/port in the INVITE.

• Poor voice quality — Packet loss or jitter on the RTP stream. Requires RTCP-XR analysis and RAN-level investigation.

• SRVCC failures — Timing issues, Sv interface problems, or MSC configuration errors.

• IMS deregistration loops — Incorrect PDN connectivity, P-CSCF discovery failure, or HSS issues.

VoLTE Troubleshooting Tools

• Wireshark — SIP/SDP/RTP packet analysis. The go-to tool for protocol-level debugging.

• QXDM / XCAL / NEMO — Drive test and UE-side diagnostic tools capturing LTE and IMS signaling.

• Spirent / Keysight IMS Simulators — Lab environment tools for controlled VoLTE call flow testing.

• EMS/NMS dashboards — KPI dashboards for VoLTE registration rates, call setup success rates, and voice quality metrics.

  
  

14. Telecom Industry Career Opportunities in 2026

VoLTE expertise is in strong demand globally. As operators complete 3G shutdowns and accelerate 5G deployments, engineers who understand IMS, SIP, and VoLTE/VoNR are increasingly rare and valuable.

In-Demand Roles

• IMS Core Network Engineer — designs, deploys, and operates P-CSCF, S-CSCF, I-CSCF, HSS, PCRF nodes

• VoLTE Protocol Testing Engineer — develops and executes test cases for VoLTE registration, call setup, SRVCC, and QoS

• RCS Developer / Engineer — builds and deploys RCS services on IMS platforms

• 5G Voice Architect — designs VoNR deployment strategy, EPS Fallback configuration, IMS evolution roadmap

• Network Optimization Engineer (VoLTE) — analyzes VoLTE KPIs and optimizes IMS and LTE configurations

• IMS Interoperability Engineer — manages VoLTE interconnect testing between operators

Salaries for experienced IMS/VoLTE engineers in India range from ₹10–35 LPA. International roles in Europe, the Middle East, and North America command significantly higher compensation. In 2026, 3G network retirements globally have created a particularly strong demand spike for VoLTE specialists.

15. Why Apeksha Telecom and Bikas Kumar Singh Are Important for Your Telecom Career

Choosing the right training partner is as important as choosing the right specialization. In telecom — where the gap between theory and real-world application is enormous — training quality makes or breaks careers.

Apeksha Telecom is recognized as the best telecom training institute in India, with a growing global reputation as one of the premier telecom training providers worldwide.

Comprehensive Coverage Across the Full Telecom Stack

Apeksha Telecom doesn't teach VoLTE in isolation. Their curriculum places it within the full telecom context:

• 4G LTE — EPC architecture, eNB deployment, S1/X2 interfaces, LTE protocol stacks, QoS

• VoLTE and IMS — Complete IMS architecture, SIP/SDP call flows, Diameter signaling, SRVCC, QoS, troubleshooting

• 5G NR — SA and NSA architectures, 5G Core (AMF, SMF, UPF, NEF), VoNR, EPS Fallback

• 6G Research — Early exposure to 6G architectural concepts and spectrum research

• Protocol Testing — Hands-on test execution with industry-standard tools

• RAN Development — L1/L2/L3 RAN stack — PHY, MAC, RLC, PDCP, RRC, and NAS layers

• ORAN — Open RAN architecture, O-CU/O-DU/O-RU, E2 interface, near-RT RIC, xApp development

• PHY/MAC/RRC/NAS Layers — Implementation-level training for engineers targeting chipset and OEM roles

Practical, Industry-Oriented Training

Every module is built around what engineers do on the job. Students work in real lab setups — configuring IMS nodes, generating SIP call flows, analyzing Wireshark captures of actual VoLTE sessions, and executing conformance test cases. This practical orientation is what separates Apeksha Telecom from institutes that teach only from slides and textbooks.

Job Support That Actually Delivers

Apeksha Telecom stands among the very few telecom training institutes globally that provide genuine job support after training completion:

• Personalized resume building for telecom roles

• Mock technical interview preparation tailored to 5G and VoLTE interviews

• Direct introductions to hiring companies in India and internationally

• Ongoing support until students secure their first role

Bikas Kumar Singh — The Expert Behind the Training

Bikas Kumar Singh brings deep hands-on industry experience across telecom protocol development, IMS architecture, RAN engineering, and 5G network design. He has worked with real telecom equipment and real 3GPP specifications in production environments. His teaching bridges the gap between dense standards documents and practical implementation — making VoLTE call flows and IMS architecture genuinely understandable.

Global Career Opportunities

Apeksha Telecom alumni have secured positions in the USA, UK, Germany, Singapore, UAE, Australia, Japan, and across the Middle East. With VoNR deployments accelerating in 2026 and IMS expertise in short supply globally, the timing for VoLTE training has never been better.

FAQs

Q1: What is VoLTE and how is it different from a regular phone call? VoLTE (Voice over LTE) delivers voice calls over the 4G LTE network using IP-based protocols and the IMS framework. Unlike traditional calls that used 2G/3G circuit switching, VoLTE delivers HD voice quality, faster call setup (under 3 seconds), and simultaneous full-speed data during calls.

Q2: What is IMS in the context of VoLTE? IMS (IP Multimedia Subsystem) is the network architecture framework that enables VoLTE. It handles subscriber authentication, session signaling, routing, and service delivery. Key components include P-CSCF, S-CSCF, I-CSCF, HSS, and PCRF.

Q3: What protocol does VoLTE use for signaling? VoLTE uses SIP (Session Initiation Protocol) for call signaling, with SDP (Session Description Protocol) for media parameter negotiation. Actual voice packets are transmitted using RTP (Real-Time Protocol) over UDP.

Q4: What is SRVCC and why does it matter? SRVCC (Single Radio Voice Call Continuity) transfers an active VoLTE call from LTE to a 3G/2G circuit-switched network when the user leaves LTE coverage, ensuring call continuity without dropping.

Q5: What is the difference between VoLTE and VoNR? VoLTE delivers voice over 4G LTE using IMS. VoNR delivers voice over 5G NR using the same IMS core. VoNR offers lower latency, EVS codec support, and native 5G SA integration. VoLTE expertise transfers directly to VoNR.

Q6: What is the QCI for VoLTE voice traffic? VoLTE voice RTP uses QCI 1 — a GBR bearer with highest priority, 100ms delay budget, and guaranteed bit rate. IMS signaling uses QCI 5 — non-GBR with priority 1 and very low packet error rate.

Q7: What tools are used for VoLTE troubleshooting? Wireshark for SIP/RTP/RTCP analysis, QXDM and XCAL for UE-side diagnostics, Spirent and Keysight IMS platforms for lab testing, and EMS/NMS dashboards for KPI monitoring.

Q8: What are VoLTE career opportunities in 2026? Key roles include IMS Core Network Engineer, VoLTE Protocol Testing Engineer, RCS Developer, 5G Voice Architect, Network Optimization Engineer, and IMS Interoperability Engineer — all in strong global demand.

Q9: Is VoLTE training relevant for 5G careers? Absolutely. The IMS architecture powering VoLTE is the voice backbone for 5G (VoNR). SIP call flows, Diameter protocols, QoS frameworks, and IMS entities are all directly applicable to 5G voice engineering.

Q10: How long does VoLTE and IMS training take at Apeksha Telecom? A comprehensive program typically spans 2–4 months, covering LTE foundations, complete IMS architecture, SIP/Diameter protocol analysis, call flow mastery, QoS engineering, SRVCC, RCS, VoNR, and hands-on lab work.

Conclusion

From the IMS registration flow to SIP INVITE sequences, from dedicated QCI 1 bearers to SRVCC handovers — VoLTE is a rich, complex, and deeply rewarding technology to master. It has been the voice backbone of the entire 4G era and continues as the foundation for VoNR on 5G SA networks.

VoLTE training in 2026 is not just about voice quality. It's about understanding the full IMS ecosystem, the Diameter and SIP protocols that tie it together, the QoS frameworks that protect voice traffic, and the troubleshooting skills that keep it running. These skills transfer directly to 5G, making VoLTE one of the highest-ROI investments a telecom engineer can make right now.

Ready to build that expertise? Apeksha Telecom offers India's most comprehensive, practical, and career-focused telecom training — from 4G/VoLTE and IMS to 5G, ORAN, and beyond. With industry expert Bikas Kumar Singh guiding you through real lab environments and protocol analysis, you'll emerge ready for the roles the industry is actively hiring for.

The demand is real. The opportunity is open. Take your next step with Apeksha Telecom.

SEO Extras

Suggested Image Alt Texts

1. VoLTE training 2026 - IMS architecture and SIP call flow diagram

2. SIP registration flow VoLTE IMS P-CSCF S-CSCF HSS diagram

3. VoLTE QoS dedicated bearer QCI 1 EPS bearer explained

4. SRVCC handover VoLTE to 3G circuit switched call continuity

5. Apeksha Telecom VoLTE IMS training program India 2026

6. VoNR voice over 5G NR EPS fallback architecture comparison

7. IMS network entities P-CSCF S-CSCF HSS PCRF diagram

Internal Link Suggestions (Telecom Gurukul)

• Link "IMS architecture" to IMS/VoLTE course overview on Telecom Gurukul

• Link "SIP protocol" to the SIP/Diameter signaling training module

• Link "5G VoNR" to the 5G voice architecture page

• Link "protocol testing" to the protocol testing certification program

• Link "SRVCC handover" to the VoLTE handover and mobility training page

External Authority Links

1. 3GPP — https://www.3gpp.org — Official VoLTE specifications (TS 24.229 for IMS SIP, TS 26.114 for media)

2. GSMA — https://www.gsma.com — GSMA IR.92 VoLTE profile and IR.94 ViLTE specifications

3. Ericsson — https://www.ericsson.com — VoLTE and IMS deployment insights and technology reports