Voice over LTE

'Voice over LTE' or VoLTE is a type of VoIP (Voice over Internet Protocol) calling that allows voice calls to be made over the 4G/LTE (Long-Term Evolution) mobile networks.

VoLTE Settings - Android 12 - Xperia 1 II

Unlike with 2G & 3G, 4G/LTE & 5G/NR are Data only standards and have no built-in calling functionality, let alone one as well standardised as the traditional Circuited Switched calling from 2G & 3G (GSM/UMTS). This poses an issue for consumers as to enable calling over 4G & 5G, devices need to have explicit carrier software/firmware support, including for Emergency Calling. Two identical phones running different carrier software can have different VoLTE Calling and Emergency Calling functionality on various networks.

Voice over LTE aka ‘4G Calling’ is a software/firmware based calling solution for mobile sector. The world has used VoIP Calling for decades but introducing VoIP data based calling into the mobile sector has proven very difficult. Voice Over LTE (4G Calling) devices have been around since as early as 2013, however over many years the industry has failed to ensure interoperability.

With 2G or 3G still being in service consumers don’t experience any loss in device functionality by switching to a competing provider. However, with the shutdown of 2G/3G services consumers may no longer be able to make or receive standard phone calls on their existing 4G device with their preferred mobile carrier. This also impacts Emergency Calls and Roaming Calls, not all devices with VoLTE Calling support can place Emergency Calls or Roaming Calls over 4G.

Due to the compatibility and standardisation issues with VoLTE, the ability for consumers to use devices purchased from any provider in the world no longer exists like it did with 2G and 3G (Circuit Switched) networks.

Currently phone carriers are able to use this telco Specific VoLTE firmware requirement as a stealth way of network locking their devices and preventing customers from switching to competitors or from using ‘BYO’ devices. Handset makers & network operators also engage in behaviour that restricts the use of devices on different networks and behaviour that otherwise undermines competition.

Device Compatibility Issues

VoLTE has been supported with Qualcomm Snapdragon Chipsets since at least 2013/2014 (e.g. Snapdragon 800/801), however not all Android devices sold have VoLTE enabled in software by the manufacturer. In many instances devices will also lack the correct carrier network configurations/profiles to support VoLTE calling on all networks.

Unlike calling with 2G and 3G there is no single configuration for VoLTE that all devices and networks universally support. Some networks support the GSMA IR.92 'Open Market Device' Configuration. This configuration is intended to be a generic/global VoLTE configuration that can be used by Open Market (non-carrier) devices. 4G/LTE Devices that lack native VoLTE calling support are reliant on 2G/3G Networks to make or receive calls (via Circuit Switched Fallback - CSFB) and without either 2G or 3G Networks those devices do not have call service.

A device may be marketed as having VoLTE support, however the device may only be able to make calls on some networks due to varying network configurations and VoLTE standardisation issues.

In some instances the retail versions of devices have been sold without VoLTE Calling on any networks (depending on brand/market/region). However the exact same device purchased directly from a carrier (with carrier software) will support VoLTE on that carrier network. Equally some handsets purchased from another carrier or from another market may not be configured (in software) to support VoLTE on all networks within a given country. This issue primarily affects Android Phones and non-Apple devices. The iPhone 6 (2014) with iOS 10 and newer support VoLTE calling on most networks, the iPhone 5 and 5s though 4G/LTE devices do not support VoLTE calling.

For example, for an Android device to have VoLTE Calling on the Telstra network the device needs to be running a Telstra Modem Configuration. Devices that are running the GSMA 'Open Market Device' configuration or a configuration from another carrier cannot get working VoLTE Calling on the Telstra network. This limitation prevents Telstra customers from using Open Market Devices which do not have native Firmware support for the Telstra Network. Open Market configuration devices can work on competing Australian providers Optus & Vodafone.

With some Qualcomm based Android devices it is possible to modify the device firmware with special software and manually load a compatible carrier modem configuration. By modifying the firmware customers can use VoLTE calling, however the carrier may not recognise those devices as supported despite working correctly.

For example, starting in May 2024 Telstra starting forcing a pre-recorded message with all outbound calls advising affected customers of the need to upgrade in advance of the 3G shutdown in August. However even customers with modified devices with working VoLTE calling on Telstra were hearing the message every time they made a 4G VoLTE call. This is due to Telstra relying on established lists of 'compatible' devices they sold (i.e. Device IMEI/TAC codes).

File:Sony-Xperia-XZ1-Compact Android-9 Phone-Info-IMS-Status-VoLTE.jpg

Android IMS Status for Voice over LTE

Android users are able to confirm if VoLTE is working by checking for the Device 'IMS Status' within a hidden Radio Info Debug menu.

If the IMS Status shows "IMS Registration: Registered" and "Voice over LTE: Available" then VoLTE is enabled and working. An IMS Status of "Not Registered" and "Voice over LTE: Unavailable" indicates VoLTE is not enabled or working. However the IMS Status debug does not confirm working 4G Emergency Calling support.

For VoLTE calling to be "Available" on a device, VoLTE has to be both provisioned/enabled in the Firmware and a carrier compatible modem configuration/profile must be loaded on the device. Typically the modem configurations are automatically loaded by the device firmware when inserting a sim card. However not all devices are configured to detect the sim cards for all networks and enable VoLTE Calling.

The IMS Status debug can also indicate if Wi-Fi Calling (Voice over Wi-Fi) and Video Calling (ViLTE/Video Telephony) are available.

(Note: For Wi-Fi Calling to say Available the device has to be connected to a suitable Wi-Fi Network).

Additionally, devices with a "UT Interface" status of "Unavailable" will be unable to change Call Forwarding or Call Busy Settings over 4G/LTE.

Note: UT Interface is not required for VoLTE Calling, UT Interface is only required to change supplementary service settings (i.e. call forwarding) on a 4G/LTE only network. Devices without an "Available" UT Interface are reliant on 2G/3G networks to change call forwarding/busy settings.

VoLTE Emergency Calling

Not all devices that have been sold as 'VoLTE capable' support making calls to Emergency Numbers over 4G/LTE with VoLTE. Some carriers and manufacturers have disabled the ability for phones to call Emergency Services with VoLTE and the devices are reliant on 2G or 3G Networks to make calls to Emergency Services.

In 2022 at the European Emergency Number Association (EENA) Conference Telecoms Expert Rudolf van der Berg made a presentation to the Conference outlining serious compatibility issues with VoLTE Calling and Emergency Calling. One of the major issues he raised is that many 4G/LTE Phones (both European & International) are completely unable to call (911/112) Emergency services without the presence of 2G/3G (Circuit Switched Calling) Networks.

These issues aren’t well known to the general public however in his EENA presentation he said these issues are “Common Knowledge” in the industry and that “..there is nobody who feels responsible to fix this”.^(16:45)

On a slide in his presentation it says ‘telecom sector will deny [there’s a problem], be angry over 5G investment and bargain for half baked measures..’

A device can have working VoLTE Calling (IMS Registration) on a network but may not be able to successfully make calls to emergency numbers over 4G. For Android devices there are methods to test a device for 4G Emergency Calling support, however such testing can require third-party software and tools to perform. Simply dialing an Emergency Number on a 4G device is not sufficient to test if a device can make VoLTE Emergency Calls.

Depending on signal strength and other factors a device may default to 2G or 3G networks (if available). Furthermore, a 4G/LTE device may say “Emergency Calls Only” within the system settings or notifications area but that message does not prove the device can actually make an Emergency call over 4G/LTE. Additionally a device may be configured for 4G Emergency Calls in a given region but fail to connect when placing a 4G Call in another country or on another network. These devices will generally get stuck on 'calling' and the call will never go through, even though a 2G or 3G network is available.

In March 2024 prior to the Switch-off of 3G Networks in Australia, the Federal Government announced that more than 740,000 4G VoLTE enabled phones would no longer be able to call Emergency Services after the 3G Networks were switched off. In early April 2024 this number was increased to more than 1 million devices.

Telstra was originally set to switch off their 3G Network on 30 June 2024. Telstra extended their switch off to 31 August in early May 2024. On 14 August 2024, Telstra and Optus further extended the 3G switch off date to 28 October 2024. Vodafone completed their 3G switch off in early January 2024.