VoLTE in Routers and IoT Devices

Key point

Most LTE modules used in industrial routers and IoT devices – particularly Cat-M1, NB-IoT, and low-cost Cat 1 modules – do not include the IMS firmware stack required for VoLTE. VoLTE in an industrial device requires a module that explicitly lists IMS/VoLTE support, plus a SIM provisioned with ISIM credentials.

Module compatibility

VoLTE in an IoT or router application requires a cellular module with:

IMS stack in firmware (handles SIP REGISTER and INVITE)
SIP client capability
Dedicated QCI 1 GBR bearer support
AMR-WB codec support in the audio path
Audio interface (PCM, I2S, or USB audio) to connect a voice endpoint

Module category	Typical examples	VoLTE support	Notes
Cat 1 bis (voice-capable)	Quectel EC21, EC25, EG25	Yes – with IMS firmware	Must confirm IMS option in firmware
Cat 4 / Cat 6 (voice-capable)	Quectel EC200A, SIMCom SIM7600	Yes – with IMS firmware	Commonly used in PSTN replacement gateways
Cat-M1 (LTE-M)	Quectel BG95, BG77, u-blox SARA-R4	No – data only	No IMS stack; designed for IoT data
NB-IoT	Quectel BC66, BC660K	No	Data only; no voice bearer
5G SA modules	Quectel RM520N, Fibocom FM350	VoNR capable (when network available)	Requires 5G SA network

PSTN replacement gateways

A class of device specifically designed for PSTN replacement uses a cellular module with VoLTE capability and presents an analogue RJ11 interface to existing telephony equipment. These cellular ATA (Analogue Telephone Adapter) style devices are the primary use case for VoLTE SIMs in the B2B market.

Cellular PSTN Replacement – System Overview

Common applications using cellular PSTN replacement gateways with VoLTE:

Lift emergency phones – BS EN 81-28 requires two-way communication; VoLTE via cellular is the standard replacement for PSTN connections
Intruder alarm diallers – registered alarm systems require an ARC-connected communicator; voice-path diallers need VoLTE
Fire panel voice communicators – some Grade D systems use voice reporting; cellular VoLTE replaces the PSTN link
Lone worker devices – personal emergency devices use voice calls to response centres
Telecare and PERS units – vulnerable person alarm systems that trigger voice calls to care lines

Teltonika and VoLTE

Teltonika Networks produce industrial cellular routers, some of which include voice capability via compatible LTE modules. The TRB245 and TRB500 devices, for example, include audio interfaces and can support VoLTE where the module firmware includes the IMS stack and the SIM is appropriately provisioned.

For any Teltonika device being used for voice: confirm the specific module variant supports VoLTE (not all module configurations do), confirm the firmware has IMS enabled, and test with a VoLTE-provisioned SIM before deployment. A standard IoT data SIM will not work.