VoLTE is how voice calls travel over 4G, and for IoT devices that make calls it is becoming the only cellular option. UK 3G networks have closed, 2G switch-off starts in 2029, and any lift phone, alarm or telecare unit that still calls over 2G has a fixed working life.

VoLTE (Voice over LTE) is a voice service that carries calls as data packets over a 4G LTE network instead of the separate circuit-switched voice channels that 2G and 3G used. The network handles VoLTE calls through the IMS (IP Multimedia Subsystem), which is the part of the operator's core network that sets up, manages and ends voice and video calls over IP.

For IoT, VoLTE matters wherever a device needs to speak to a person: lift emergency phones, alarm and intercom panels, telecare units, door entry systems and lone worker devices. It also matters for phones and in-vehicle voice used by drivers and mobile workers who cannot afford to lose calls when one network drops out.

This guide explains how VoLTE works in IoT devices, which cellular technologies support it, what a device and SIM need before VoLTE calls will connect, and where VoNR (voice over 5G) fits.

Why Does VoLTE Matter for IoT Devices?

VoLTE matters for IoT devices because once 2G and 3G networks close, a cellular device can only make a standard voice call over 4G or 5G if it supports VoLTE or VoNR.

A 4G device does not always make its calls over 4G. Devices without VoLTE use CSFB (circuit-switched fallback), which is a method where a 4G device drops back to a 2G or 3G network each time it places or receives a call, then returns to 4G when the call ends. CSFB works only while a 2G or 3G network is there to fall back to.

The UK has already switched off 3G. When 2G goes, a device that relies on CSFB will still show a 4G connection and still pass data, but it will not be able to make or receive ordinary voice calls. For a lift phone or a telecare alarm, that failure may not show up until someone presses the button.

Ericsson notes that circuit-switched fallback is not standardised for 5G, which makes VoLTE a requirement for operators moving to 5G Standalone networks. VoLTE is the voice layer that the rest of the network roadmap is built on.

When Do UK 2G Networks Switch Off?

UK 2G networks switch off between 2029 and 2033: EE begins in May 2029, O2 begins in summer 2029, VodafoneThree switches off the Vodafone 2G network during 2030, and every operator has committed to finish by 2033.

Ofcom's advice to IoT and third-party device suppliers, updated on 1 September 2026, says all 2G-only devices need upgrading to at least 4G, and warns that finding and sourcing suitable replacement devices can take time.

The PSTN date sits in the same table because it affects the same devices. BT lists alarms, lift lines, telecare equipment and door entry systems among the equipment to review before its copper network closes on 31 January 2027. Many of those devices are moving from a landline to a cellular connection now. A device fitted today should use VoLTE, or it will need replacing again when 2G closes.

How Does VoLTE Work?

VoLTE works by treating a phone call as a data session: the device registers with the operator's IMS, sets up the call using SIP signalling, and sends the voice as IP packets over the 4G connection.

SIP (Session Initiation Protocol) is the signalling standard used to start, change and end calls over IP networks. VoLTE follows a GSMA profile built on the 3GPP IMS multimedia telephony standard, according to Ericsson. That shared profile is what lets a VoLTE call work across different operators, and reach ordinary phone numbers anywhere.

Ericsson notes that VoLTE supports HD voice and video calling. A VoLTE device can also carry voice and data at the same time, so an IoT panel can stay on a call while it still reports status or receives remote management commands.

How Is VoLTE Different from VoIP?

VoLTE and VoIP (Voice over IP) both carry voice as data, but VoLTE runs inside the mobile operator's network and IMS, while VoIP runs over any internet connection to a separate call platform. VoIP is the more universal option because it works over any IP data connection. VoLTE gives HD voice through the mobile network's own call handling, but only where the operator supports it.

The right choice depends on the device, the networks it will use, and whether it must reach emergency services. We cover the trade-offs in detail in our guide to VoLTE versus VoIP for IoT.

Which IoT Technologies Support VoLTE?

VoLTE is supported on LTE-M, LTE Cat 1, LTE Cat 1 bis and higher LTE categories such as Cat 4 and Cat 6, but not on NB-IoT.

LTE-M is a low-power version of 4G designed for IoT devices that move or need modest data rates. NB-IoT is a narrowband 4G technology for small, infrequent data transfers, and it has no voice capability. LTE Cat 1 bis is a single-antenna version of LTE Cat 1, used where a device needs a small, low-cost 4G modem. Our guide to 4G and 5G LTE categories compares these in more detail.

Ericsson notes that Cat M1 modules provide basic voice for alarm panels, wearables and digital locks. However, VoLTE support on a technology does not mean every module on that technology supports it, so check the module before you choose the hardware.

What Does an IoT Device Need to Make VoLTE Calls?

An IoT device needs four things in place before VoLTE calls will connect: a module that supports VoLTE, firmware configured for the network, a SIM and tariff with voice and VoLTE enabled, and a network that accepts VoLTE from that device.

1. A VoLTE-capable module. The cellular module inside the device must support VoLTE in hardware and firmware. Check the module datasheet as well as the device's product sheet.
2. Up-to-date firmware. The module firmware needs an IMS profile that matches the operator's VoLTE settings. We have often had to update the firmware on phones before VoLTE would work on our SIMs, and a firmware fault can stop a device connecting at all.
3. A SIM and tariff with voice and VoLTE enabled. Many IoT SIMs are data-only. A voice device needs a voice-enabled tariff, and VoLTE must be switched on for that SIM on each network it will use.
4. Network acceptance. VoLTE support varies by operator and by country, so check support on every network the device will use. Some operators also restrict VoLTE to devices they have tested or approved.

Can a VoLTE Device Make Emergency Calls?

A VoLTE device can make emergency calls when both the device and the network support emergency calling over 4G, but this needs testing rather than assuming. Mobile UK states that 999 calls automatically roam onto any network provider. That describes how the networks treat 999 calls, not whether a particular IoT device supports emergency calling over 4G. If a device must reach 999 or 112, confirm emergency calling with the device manufacturer and test it as part of commissioning.

Which IoT Applications Use VoLTE?

VoLTE is used in IoT wherever a device must connect a person to another person: lift emergency phones, alarm and intercom panels, telecare and personal alarms, door entry, lone worker devices, and phones or in-vehicle voice for drivers and mobile workers.

Lift Emergency Phones

Lift emergency phones use VoLTE to connect a trapped passenger to a rescue service over 4G. Many lift lines are moving off copper ahead of the PSTN switch-off, and a cellular lift phone fitted now should support VoLTE so it does not depend on 2G.

Alarms, Intercoms and Door Entry

Alarm and intercom panels use VoLTE for two-way speech with a monitoring centre, a keyholder or a visitor at the gate. Door entry systems use it to call a resident or site manager's phone.

Telecare and Lone Worker Devices

Telecare units and lone worker devices use VoLTE so the wearer can speak to a response centre when they raise an alert. These devices often run on LTE-M or Cat 1 bis to save battery and space, so check VoLTE support at module level before choosing one.

Drivers, Mobile Workers and Multi-Network Resilience

Multi-network roaming SIMs with VoLTE enabled suit phone and voice uses where losing one network must not stop calls, such as taxi and private hire drivers, field engineers and delivery crews. A multi-network roaming SIM is a SIM that can connect to more than one UK mobile network, so the device can register on another network if its current one has no coverage.

For a driver, that matters because work arrives by phone and a journey can pass through places where one network is weak. A single-network SIM leaves the phone without service wherever that network has no coverage. A multi-network SIM gives the phone more networks to connect to, as long as voice works on each of them. The same logic applies to any voice device that moves, or that sits somewhere no single network covers well.

VoLTE must be enabled for the SIM on each network it can use. If VoLTE works on one network but not another, calls on the network without it fall back to 2G where the device and network allow, and fail where they do not, even though data carries on. Ask your SIM supplier which networks carry VoLTE on the tariff.

How Do Millbeck Multi-Network SIMs Handle Voice?

Millbeck multi-network roaming SIMs carry VoLTE on EE, O2 and Three. On Vodafone, calls on these SIMs use 2G instead, so they work only for devices with a 2G radio and only until Vodafone switches off its 2G network during 2030.

For a driver's phone or an in-vehicle voice device, the practical choice is a device that supports both VoLTE and 2G, which can make calls on all four networks today. A 4G-only or LTE-M device cannot make calls while registered on Vodafone. Millbeck supplies IoT SIMs for single-network and multi-network use.

What Is VoNR?

VoNR (Voice over New Radio) is voice over 5G: it carries calls natively on a 5G Standalone network, where VoLTE carries them on 4G.

5G Standalone (SA) is a 5G network with its own 5G core. Ericsson describes VoNR as the third step in the evolution of 5G voice, carrying voice, video calling and messaging over 5G access. Ericsson also notes that VoNR needs continuous 5G coverage, so early deployments hand calls over to VoLTE when a device moves outside it.

Before VoNR, a 5G device places calls in one of two ways. On 5G Non-Standalone networks, which rely on a 4G core, calls use VoLTE. On 5G Standalone networks without VoNR, the device uses EPS fallback, which is a method where the network moves the device from 5G to 4G for the length of the call so it can use VoLTE. Both paths still depend on VoLTE.

Is VoNR Available in the UK?

VoNR is available in the UK on EE, which switched on voice over 5G Standalone for compatible smartphones in June 2025, according to 5g.co.uk. Coverage depends on 5G Standalone reaching the site, and calls move to VoLTE outside it. Millbeck can supply voice and data SIMs on EE with VoNR access where a project needs it.

Does VoNR Matter for IoT Yet?

VoNR does not yet matter for most IoT voice devices, because VoLTE will carry IoT voice for years and only one UK network offers VoNR today. For a device fitted now, VoLTE support is the requirement. VoNR is worth asking about only where a device already runs on 5G Standalone.

What Should You Do About IoT Voice Devices Now?

If you run IoT devices that make calls, find every device that still relies on 2G or 3G for voice, confirm VoLTE at module, SIM and network level, and test calls before the next site visit.

1. Audit the estate. List every device that makes or receives calls, its module, its SIM and the networks it uses.
2. Find the CSFB devices. A 4G device that falls back to 2G for calls will lose voice when 2G closes, even though it looks like a 4G device.
3. Check roaming and multi-network SIMs first. O2 has already withdrawn 2G for inbound roaming, so these devices may have lost voice on one network already.
4. Specify VoLTE for every new voice device. Include devices moving off copper ahead of the PSTN switch-off.
5. Test real calls on every network. Include emergency calls where the device must reach 999 or 112.

For replacement voice devices, Millbeck can supply IoT SIMs with voice and VoLTE enabled on the networks you need, and help you test calls before the devices go on site.

Frequently Asked Questions

Sources and Further Reading

• Ofcom: Switching Off 2G and 3G Networks, Advice for IoT and Third-Party Device Suppliers (updated 1 September 2026)
• Ofcom: Switching Off the UK's 2G and 3G Mobile Networks (updated 10 August 2026)
• BT Business: The UK's PSTN Network Will Switch Off in 2027
• Mobile UK: 2G Switch Off Q&A
• Ericsson: Voice and Communication Services in 4G and 5G Networks (white paper)
• 5g.co.uk: Voice Over 5G Standalone (VoNR)