VoLTE Glossary

Plain-English definitions for VoLTE, IMS, 5G voice and the rest of the alphabet soup behind mobile calls, based on 3GPP and GSMA standards. Every term has its own link, so you can share a single definition, and related terms link to each other.

81 terms. Updated October 2026.

0-9

3GPP

3rd Generation Partnership Project. The standards body that writes the specifications for 2G through to 5G, including IMS, VoLTE and VoNR. Specifications are numbered TS xx.xxx and grouped into Releases.

4G Calling

The consumer name EE, O2, Vodafone and Three use for VoLTE. If your network says 4G Calling, it means VoLTE. More: VoLTE problems and fixes.

5G Calling

EE’s name for VoNR, launched in June 2025 in 5G Standalone areas. Needs 5G SA coverage and a compatible phone and plan. More: VoNR guide.

5G NSA

5G Non-Standalone. 5G radio added on top of a 4G network, with LTE as the anchor. Calls use VoLTE over the 4G side, so broken VoLTE means broken calls on 5G NSA too. More: VoLTE and 5G.

5G SA

5G Standalone. 5G radio connected to a 5G core with no 4G anchor. Makes VoNR possible. Where VoNR isn’t available, calls use EPS Fallback to VoLTE.

5GC

5G Core. The core network for 5G Standalone, replacing the EPC. Its main functions include the AMF (access and mobility), SMF (sessions), UPF (user data) and PCF (policy).

5QI

5G QoS Identifier. The 5G equivalent of QCI. 5QI 1 carries conversational voice and 5QI 5 carries IMS signalling, mirroring VoLTE.

A

AMR-NB

Adaptive Multi-Rate Narrowband. The classic mobile voice codec: audio from 300 Hz to 3,400 Hz at 4.75 to 12.2 kbps. Used on 2G and 3G, and still used on VoLTE when the other end of a call can’t do wideband.

AMR-WB

Adaptive Multi-Rate Wideband (ITU-T G.722.2). The codec behind HD Voice: audio from 50 Hz to 7,000 Hz at 6.6 to 23.85 kbps, with 12.65 kbps the most common mode on VoLTE. Mandatory for VoLTE devices under GSMA IR.92.

APN

Access Point Name. Tells the network which packet network a device wants to reach. A VoLTE device normally uses at least two: an internet APN for data and an IMS APN for calls.

ARC

Alarm Receiving Centre. A monitoring centre that receives signals from intruder, fire and personal alarms. Signals arrive over IP or as tones over a voice call, such as Contact ID, which must be tested end to end over VoLTE. More: PSTN replacement.

ATA

Analogue Telephone Adapter. Converts an analogue phone line (RJ11) to IP or cellular voice. Widely used in PSTN replacement for lift lines, alarm diallers and door entry. On cellular, it needs a modem and SIM that support IMS voice. More: Routers and IoT.

C

Contact ID

An alarm reporting protocol that sends event codes as DTMF tones over a voice call. It can be unreliable over VoLTE because tones are carried differently from speech, so always test with the real receiving centre.

CS and PS

Circuit switched and packet switched. 2G and 3G voice reserved a dedicated circuit for each call (CS). Everything on 4G and 5G is packets (PS), which is why voice needs IMS.

CSCF

Call Session Control Function. The family of SIP servers at the heart of IMS: the P-CSCF, I-CSCF, S-CSCF and E-CSCF.

CSFB

Circuit Switched Fallback. A 4G device without VoLTE drops to 2G or 3G for every call. With all UK 3G now closed (O2 was last, in early 2026), CSFB lands on 2G only, and it stops working altogether as 2G closes between 2029 and 2033. Tell-tale signs: E or G on screen during calls and data stalling. More: Why phones drop to 2G.

D

Digital Voice

Home phone service delivered over broadband instead of an analogue line, as the UK PSTN closes. Many of these services run on IMS cores, the same technology behind VoLTE. More: PSTN replacement.

DSDS

Dual SIM Dual Standby. Two SIMs or eSIMs in one phone sharing a radio. VoLTE has to be supported and enabled per line, so check each SIM separately.

DTMF

Dual-Tone Multi-Frequency. The tones your keypad makes. On VoLTE they’re usually sent as signalled events (RFC 4733) rather than as sound, which matters for phone menus, door entry and alarm diallers.

E

E-CSCF

Emergency CSCF. Routes IMS emergency calls to 999 and 112 to the right emergency centre, together with location information. More: Emergency calls over VoLTE.

eCall

The in-vehicle emergency call system. First-generation eCall was built on 2G and 3G circuit-switched calls; next-generation eCall uses IMS.

ePDG

Evolved Packet Data Gateway. The operator gateway that terminates the encrypted tunnel from a phone on Wi-Fi, connecting Wi-Fi Calling to the packet core and IMS.

EPC

Evolved Packet Core. The 4G LTE core network: the MME (mobility), SGW (serving gateway), PGW (packet gateway), HSS and PCRF. The 5G equivalent is the 5GC.

EPS bearer

A path with defined quality of service between a device and the LTE core. VoLTE uses a default bearer for IMS signalling (QCI 5) and a dedicated GBR bearer for the voice itself (QCI 1).

EPS Fallback

How a 5G SA network handles a call when VoNR isn’t available: the phone moves to 4G, the call is set up as VoLTE, and the phone returns to 5G afterwards. Seeing 5G change to 4G during a call is usually this, not a fault. More: VoLTE and 5G.

eSIM

A SIM profile downloaded into a chip in the device instead of a plastic card. VoLTE works identically on eSIM: what matters is the profile and the subscription.

eSRVCC

Enhanced SRVCC. Speeds up handing a VoLTE call to a circuit-switched network, targeting an interruption of around 300 ms. Introduced in 3GPP Release 10.

EVS

Enhanced Voice Services. The successor to AMR-WB (3GPP TS 26.441), supporting narrowband through super-wideband and fullband audio up to 20 kHz, with better resilience to packet loss. Used on VoNR and some VoLTE networks, but only when both ends support it.

G

GBR bearer

Guaranteed Bit Rate bearer. A bearer that reserves capacity on the network. VoLTE voice uses a GBR bearer with QCI 1 so audio isn’t squeezed out by other traffic.

GSMA

The industry association of mobile operators. Its Permanent Reference Documents, such as IR.92, profile 3GPP standards so devices and networks work together. More: Standards.

H

HD Voice

The marketing name for wideband voice, normally AMR-WB. Not a separate standard. Both ends, and every network in between, must support wideband or the call drops to narrowband.

HSS

Home Subscriber Server. The subscriber database for LTE and IMS: identities, authentication data and service entitlements, including whether the account is allowed IMS voice at all. Its 5G counterpart is the UDM.

I

I-CSCF

Interrogating CSCF. The entry point into a subscriber’s home IMS. Asks the HSS which S-CSCF should serve a registration or call, and routes it there.

IMPI

IMS Private User Identity. The identity used to authenticate a subscriber to IMS, never shown to other parties. Held on the ISIM where one exists; otherwise derived from the IMSI on the USIM.

IMPU

IMS Public User Identity. The address used to reach a subscriber, as a SIP URI or tel URI built around their phone number. A subscriber can have several. Held on the ISIM or derived from the IMSI, like the IMPI.

IMS

IP Multimedia Subsystem. The part of an operator’s core network that controls voice, SMS over IP and other real-time services over IP. It powers VoLTE, Wi-Fi Calling and VoNR. Architecture defined in 3GPP TS 23.228. More: How VoLTE works.

IMS APN

The APN, usually named ims, that a device uses for its separate connection to the IMS core. Distinct from the internet APN, which is why data can work while calls fail, and why changing the internet APN doesn’t fix VoLTE. More: Why data works but calls don’t.

IMS registration

The process where a device identifies itself to IMS using SIP REGISTER and SIM-based authentication. Nothing works until it succeeds. IMS not registered on a good 4G signal is the classic VoLTE fault. More: Diagnosing VoLTE.

IMSI

International Mobile Subscriber Identity. The subscriber identifier stored on the SIM. IMS identities can be derived from it when there’s no ISIM.

IR.51

GSMA profile for IMS voice, video and SMS over Wi-Fi. The VoWiFi equivalent of IR.92.

IR.92

GSMA IMS profile for voice and SMS over LTE. Defines the minimum features VoLTE devices and networks must support so they work together. More: Standards.

ISIM

IMS Subscriber Identity Module. An optional application on the SIM that stores dedicated IMS identities. Useful, but not required: many networks run VoLTE using the USIM alone. What matters is that the subscription is provisioned for IMS voice. More: VoLTE SIM cards.

L

LTE

Long Term Evolution, the 4G radio standard. An all-IP network with no circuit-switched voice, so every 4G call is VoLTE or a fallback to an older network.

LTE-M

LTE Cat-M1, a low-power LTE variant for IoT. VoLTE is possible in the standard but rarely offered by networks or supported by modules. Confirm in writing before designing a voice device around it.

M

MBN

A modem configuration file used by Qualcomm-based cellular modules, such as many Quectel models. Holds operator-specific IMS, APN and voice settings. A wrong or missing MBN is a common reason a router does VoLTE on one network but not another. More: Routers and IoT.

MGCF

Media Gateway Control Function. Connects IMS calls to the PSTN and circuit-switched networks by translating SIP into their signalling, and controls the MGW.

MGW

Media Gateway. Converts call audio between IMS (RTP packets) and circuit-switched formats, under control of the MGCF.

MMS

Multimedia Messaging Service, used for picture messages. Needs mobile data, an MMS APN and the operator’s MMSC, and is still billed per message on many tariffs. Not the same as SMS. More: SMS, MMS and RCS.

MO and MT

Mobile Originated and Mobile Terminated: outgoing and incoming calls or texts. They fail for different reasons, so always test both directions separately.

N

NB-IoT

Narrowband IoT. A low-power LTE variant for small amounts of data. It has no voice capability at all.

NG.114

GSMA profile for IMS voice, video and messaging over 5G. The VoNR equivalent of IR.92.

P

P-CSCF

Proxy CSCF. The first IMS node a device talks to. Relays SIP signalling, secures the link to the device and asks the policy function to reserve quality for each call.

PCF

Policy Control Function. The 5G equivalent of the PCRF, authorising quality of service for VoNR calls.

PCRF

Policy and Charging Rules Function. The LTE element that authorises quality of service. For each VoLTE call it triggers the GBR bearer with QCI 1.

PDN connection

The data session a device opens to an APN on 4G. VoLTE uses its own PDN connection to the IMS APN. The 5G equivalent is a PDU session.

PGW

PDN Gateway. The LTE core node that gives a device its IP address for each APN, including the IMS APN. Replaced in 5G by the UPF and SMF.

PSTN

Public Switched Telephone Network. The UK’s traditional analogue phone network, closing by 31 January 2027 as providers move customers to digital voice. Lift lines, alarms and telecare need a replacement path, often cellular VoLTE. More: PSTN replacement.

Q

QCI

QoS Class Identifier. Defines how the LTE network treats a bearer. QCI 1 (guaranteed bit rate, priority 2, 100 ms delay budget, 10^-2 packet error rate) carries VoLTE voice. QCI 5 (non-GBR, priority 1, 100 ms, 10^-6) carries IMS signalling.

R

RCS

Rich Communication Services. The IP-based successor to SMS and MMS, with read receipts, typing indicators and high-quality media. Supported on Android and, since iOS 18, on iPhone where the network enables it. Falls back to SMS and MMS when unavailable.

RTP

Real-time Transport Protocol. Carries the audio packets of a VoLTE call over UDP on the dedicated QCI 1 bearer, between the device and the operator’s network and on to the other party.

S

S-CSCF

Serving CSCF. The central IMS server for a subscriber: handles registration and authentication, applies the service profile and routes every call and message, passing feature handling to the TAS.

S8HR

S8 Home Routing. The GSMA’s preferred model for VoLTE roaming: the visited network provides the radio and core, and IMS traffic is routed back to the home network’s IMS. More: VoLTE roaming.

SBC

Session Border Controller. Protects IMS where it meets devices and other networks. Often hosts the P-CSCF and handles interconnection between operators.

SDP

Session Description Protocol. Carried inside SIP messages to agree codecs, addresses and ports for a call. Mismatches can leave a connected call silent or with one-way audio.

SGs

The interface between the LTE MME and the legacy circuit-switched core, used for CSFB and SMS over SGs.

SIP

Session Initiation Protocol (IETF RFC 3261). The signalling protocol IMS uses to register devices and set up, change and end calls. INVITE starts a call; BYE ends it.

SMS over IMS

Also called SMS over IP. Text messages sent as SIP MESSAGE requests through IMS (3GPP TS 24.341). Increasingly important as the circuit-switched core retires. A mismatch with SMS over SGs is a common cause of SMS failures on IoT modules. More: IoT VoLTE.

SMSC

Short Message Service Centre. The operator server that stores and forwards text messages.

SRVCC

Single Radio Voice Call Continuity. Hands an active VoLTE call to a circuit-switched network when 4G coverage runs out. In the UK the only remaining target is 2G, and once 2G closes there will be none, so calls leaving 4G coverage will drop.

T

TAS

Telephony Application Server. Runs phone features for IMS calls: forwarding, waiting, hold, barring, conferencing and voicemail access. Settings are managed from the phone over Ut/XCAP.

U

UE

User Equipment. The 3GPP term for any device on the network: phone, router, module or tablet.

UE usage setting

Tells the network whether a device is voice-centric or data-centric. A voice-centric device without VoLTE may leave 4G hunting for a network that offers voice; a data-centric one stays on 4G without it.

UICC

Universal Integrated Circuit Card. The smart card that holds SIM applications. A VoLTE-capable card always has a USIM and may also carry an ISIM.

UPF

User Plane Function. The 5G core element that carries user data, replacing the PGW’s data role.

USIM

Universal Subscriber Identity Module. The SIM application holding the IMSI and authentication keys. Present on every modern SIM, and enough on its own for IMS authentication on many networks.

Ut / XCAP

The interface a phone uses to manage call features such as forwarding and call waiting on VoLTE networks. It runs over the data connection, so changing those settings can fail with mobile data switched off.

V

VoIP

Voice over IP in general. VoLTE is a specific, operator-managed form of VoIP with SIM authentication, your mobile number, guaranteed priority on the radio and emergency calling.

VoLTE

Voice over LTE. Voice calls over a 4G network, controlled by IMS and defined by GSMA IR.92. Uses SIP for signalling, RTP for media, AMR-WB or EVS for audio and a QCI 1 bearer for the voice. More: VoLTE problems and fixes.

VoNR

Voice over New Radio. Voice over 5G Standalone using the same IMS core as VoLTE, profiled in GSMA NG.114. Supports EVS. Launched in the UK by EE as 5G Calling. More: VoNR guide and vonr.co.uk.

VoWiFi

Voice over Wi-Fi, marketed as Wi-Fi Calling. IMS calls over any Wi-Fi connection, through an encrypted tunnel to the operator’s ePDG. Uses the same IMS registration and subscription as VoLTE, so it can hide a VoLTE fault indoors but can’t fix an account that isn’t provisioned for IMS voice.

W

Wi-Fi Calling

The consumer name for VoWiFi.

Missing a term? Tell us and we’ll add it. For the standards behind these definitions, see our standards and regulatory page.