What Is IMS? The Hidden Network Behind VoLTE, Wi-Fi Calling and 5G Voice
Two phones, same network, same SIM type, same four bars of 4G. One makes calls perfectly. The other can browse all day but every call fails or goes...
UK Technical Authority
The definitive resource for Voice over LTE
VoLTE SIM cards, PSTN replacement, 2G and 3G sunset guidance, HD voice technical reference, and VoNR. Everything engineers and specifiers need in one place.
Quick Answer
What is VoLTE?
VoLTE (Voice over LTE) is the 3GPP-standardised method for carrying voice calls over a 4G LTE network using the IP Multimedia Subsystem (IMS). Defined in GSMA IR.92, it delivers HD audio via the AMR-WB codec and eliminates the need to fall back to 2G or 3G for voice calls.
Jan 2027
UK PSTN closure deadline
UK PSTN closure deadline
4 operators
UK networks with VoLTE live
UK networks with VoLTE live
AMR-WB
HD voice codec – 3x clarity of 3G
HD voice codec – 3x clarity of 3G
3G gone
All UK 3G networks switched off 2024
All UK 3G networks switched off 2024
The standard
What is VoLTE?
VoLTE stands for Voice over LTE. It is the 3GPP-standardised way to carry voice calls entirely over a 4G LTE network using the IP Multimedia Subsystem (IMS). Before VoLTE, a 4G phone making a voice call would drop back to 2G or 3G – a process called Circuit Switch Fallback (CSFB). VoLTE eliminates that fallback entirely.
The practical results are faster call setup (under 2 seconds vs 6-8 for CSFB), HD voice quality via the AMR-WB codec, and the ability to use data at full 4G speed during a call.
With all UK 3G networks now closed, VoLTE is not optional – it is the only way to make voice calls on a 4G-only device or SIM.
VoLTE vs Circuit Switch Fallback
Key topics
Everything on VoLTEsim.co.uk
Technical reference for engineers, installers, and specifiers who need to understand VoLTE.
VoLTE SIM Cards
Not all SIMs support VoLTE. A SIM must carry ISIM credentials and be provisioned for IMS services by the operator. Standard IoT data SIMs do not qualify.
PSTN Replacement
The UK PSTN closes in January 2027. Lifts, alarms, fire panels, and telecare devices that rely on analogue lines need a replacement path – often VoLTE via a cellular gateway.
2G and 3G Sunset
All four UK operators completed their 3G switch-offs in 2024. Devices that relied on 3G voice must migrate to VoLTE. There is no circuit-switched voice on 4G.
VoLTE in Phones
Every modern UK smartphone supports VoLTE, but activation depends on carrier provisioning, handset approval lists, and correct SIM configuration.
Routers and IoT
Industrial routers and IoT devices need specific LTE modules with an IMS stack to support VoLTE. Most Cat-M1 and NB-IoT modules do not qualify.
VoNR – What Comes Next
VoNR (Voice over New Radio) is VoLTE’s successor on 5G Standalone networks. It uses the same IMS core but delivers voice over the 5G NR air interface with the EVS codec.
PSTN closure: January 2027
BT Openreach is switching off the UK’s Public Switched Telephone Network by January 2027. Any device using an analogue telephone line – lifts, alarm diallers, fire panels, door entry, telecare – must be replaced or upgraded before that date. Cellular VoLTE is the most practical replacement for sites without fixed broadband.
BT Openreach is switching off the UK’s Public Switched Telephone Network by January 2027. Any device using an analogue telephone line – lifts, alarm diallers, fire panels, door entry, telecare – must be replaced or upgraded before that date. Cellular VoLTE is the most practical replacement for sites without fixed broadband.
Interactive explainer
Follow a VoLTE call
Pick a situation to see the route your call takes through the network, where it breaks, and how to fix it.
Press play to follow the call step by step.
Latest guides
All guides Problems, fixes and explainers
In-depth VoLTE guides for phone users, installers and engineers, from IMS registration to the 2G switch-off.
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