Voice over Long-Term Evolution (acronymVoLTE) is anLTE high-speedwireless communication standard forvoice calls andSMS usingmobile phones and data terminals.[1][2] VoLTE has up to three times more voice[3] and data capacity than older3GUMTS and up to six times more than2GGSM. It uses less bandwidth because VoLTE's packetheaders are smaller than those of unoptimizedVoIP/LTE.[citation needed] VoLTE calls are usually charged at the same rate as other calls.
To be able to make a VoLTE call, the device, its firmware, and the mobile telephone providers on each end, as well as the inter-carrier connectivity must all implement the service in the area, and be able to work together. VoLTE has been marketed as "HD voice" by some carriers, but this is a broader concept. Moreover, HD+ (EVS) is used only in LTE andNR; HD voice was available in 3G too.
VoLTE is based on theIP Multimedia Subsystem (IMS) architectural framework, with specific profiles forcontrol andmedia planes of voice service. This facilitates VoLTE on theLTE wireless broadband service defined byGSMA in PRD IR.92.[4] The approach results in the voice service (control and media planes) being delivered as data flows within the LTE data bearer, with no dependency on (or ultimately, requirement for) thecircuit-switched voice network to be in the call path.
As of February 2019 there were 253 operators investing in VoLTE in 113 countries globally, including 184 operators with commercially launched VoLTE-HD voice service in 87 countries, up from 137 operators in 65 countries 12 months previously, according to data from theGlobal Mobile Suppliers Association.[5] By August 2019, these numbers had risen to 262 operators investing in VoLTE in 120 countries and 194 operators with launched VoLTE-HD voice services in 91 countries.[6]
VoLTERoaming historically was required to make VoLTE Calls when roaming on another4G/LTE network. Now countries with only 3G cores use 3G-to-VoLTE Roaming Interworking.[7]
Devices without VoLTE Roaming cannot make VoLTE calls when roaming on another network. VoLTE Roaming uses the S8HR Architecture, this architecture routes the calls to the home mobile network or to a 3G home network using 3G-to-VoLTE Roaming Interworking .[8]Roaming devices that lack VoLTE Roaming support can use2G or3G Networks (if available) for making and receiving calls (via Circuit Switched Fallback). In countries without either2G or3G Networks, Roaming devices will not have access to call service from the connected Roaming Network.
VoLTE Roaming Support requires anAndroid Device withAndroid12 (2021) or newer or aniPhone withiOS15 (2021) or newer.[9]As of April 2023 Devices fromAndroid4 -11 made up approximately 70% of the Global Android Device Market according toGoogle.[10] These older devices are largely in demographics that are less likely to roam.[citation needed]
Not all devices that have been sold as 'VoLTE capable' support making calls to Emergency Numbers over 4G/LTE with VoLTE.[11] 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 toEmergency Services.[12]
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.[13]
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 isnot sufficient to test if a device can make VoLTE Emergency Calls.[14]
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.[14] 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. These devices will generally get stuck on 'calling' and the call will never go through, even though a 2G or 3G network is available.[14]
In March 2024 prior to the Switch-off of3G Networks inAustralia, theFederal Government announced that more than 740,0004G VoLTE enabled phones may no longer be able to call Emergency Services after the3G Networks were switched off.[15] In early April 2024 this number was increased to more than 1 million devices in the media with no change to the government reporting. After the completion of the review the number was less than 70,000 of devices that may not be able to call Emergency Services.[citation needed]
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.[16] On 14 August 2024, Telstra and Optus further extended the 3G switch off date to 28 October 2024.[17]Vodafone completed their 3G switch off in early January 2024.[18]
Evolved packet system (acronymEPS) fallback is a crucial mechanism used in early5G networks, that seamlessly shifts voice calls to the LTE network through VoLTE wheneverVoice over NR (VoNR) is not available or that the user’s equipment (UE) doesn’t support it.[19][20][21]
This mechanism in early 5G deployments is essential because expanding5G takes time and allows calls to drop to the widely available LTE network throughSingle Radio Voice Call Continuity (SRVCC) for handovers or simple redirection to LTE radio access, preventing dropped calls and maintaining Quality of Service (QoS), even if it inherently brings higher latency.[19][20][21]
There have been several issues with VoLTE deployment:
VoLTE has been supported by allQualcomm Snapdragon chipsets since at least 2014 (e.g. Snapdragon 800/801), but someAndroid device manufacturers disabled it in software.[26][27] Manufacturers of many devices omitted the carrier network configurations/profiles required to support VoLTE calling on all networks.[28]
Unlike calling with2G and3G, there is no single configuration for VoLTE that all devices and networks support. Some networks support theGSMA IR.92 'Open Market Device' Configuration.[29] This configuration is intended to be a generic/global VoLTE configuration that can be used by Open Market (non-carrier) devices.[30]4G orLTE devices that lack native VoLTE calling support are reliant on a2G or3G network to make or receive calls via circuit-switched fallback (CSFB), and will otherwise have no call service.
A device marketed as having VoLTE support may only be able to make calls on certain networks, due to varying network configurations and VoLTE standardisation issues.[13]
In some instances (depending on brand, market, or region), devices have been sold that are unable to make VoLTE calls on any network, while the same devices purchased directly from a carrier (with carrier software) support VoLTE on that carrier's network.[31] Some handsets purchased from another carrier or from another market may have software that is not configured to support VoLTE on all networks within a given country. This issue primarily affectsAndroid and other non-Apple devices. TheiPhone 6 (2014) and newer, with at leastiOS 10, support VoLTE calling on most networks. TheiPhone 5 and5s do not support VoLTE calling, even though they include LTE.[32][33][34]
For example, for an Android device to have VoLTE calling on theTelstra network, the device needs to be running a Telstra modem configuration. Devices that are running theGSMA 'Open Market Device' configuration or a configuration from another carrier cannot make VoLTE calls on the Telstra network. This prevents Telstra customers from using Open Market Devices. Open Market configuration devices can work on competing Australian providersOptus &Vodafone.[29][31]
With someQualcomm based Android devices it is possible to modify the device firmware with special software and manually load a compatible carrier modem configuration,[29] enabling VoLTE calling; a carrier may not recognise the device as supported, though it works 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.[35] This is due to Telstra relying on established lists of 'compatible' devices (i.e. DeviceIMEI/TAC codes).
Android users are able to confirm if VoLTE is working by checking for the Device 'IMS Status' within a hidden Radio Info Debug menu.[29][36]
If theIMS 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 debugdoes not confirm working 4G Emergency Calling support.[14]
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 asim card. However not all devices are configured to detect the sim cards for all networks and enable VoLTE Calling.[29]
The IMS Status debug can also indicate ifWi-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 changeCall Forwarding or Call Busy Settings over 4G/LTE. Note: UT Interface is not required for VoLTE Calling, UT Interface is only required to changesupplementary 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.
Beginning in August 2012,MetroPCS launched the world's first commercial VoLTE services inDallas, Texas, in theUnited States, alongside the first VoLTE phone, the LG Connect 4G.[37] In May 2014,Singtel introduced the world's first commercial "full-featured" VoLTE service in Singapore, only in combination withGalaxy Note 3, it was subsequently expanded.[38] In June 2014,KT showcased the world's first cross-borderroaming services based on Voice over LTE. The South Koreanoperator partnered withChina Mobile to develop VoLTE roaming services.[39]
In November 2014,Verizon andAT&T announced the companies are enabling VoLTE-to-VoLTE connections between their respective customers. VoLTE interoperability between Verizon and AT&T customers began in 2015. Testing and design were performed between both companies using third party networks such asAlcatel-Lucent.[40] This was stated to have been completed in November 2017.[41]
On July 11, 2015, SEATEL Cambodia announced the world's first commercial 100% VoLTE service without 2G/3G in Cambodia.[42][43][44][45][46]
As of 2020, almost all new phones for sale have the potential to support VoLTE.[citation needed]
On December 31, 2022,Verizon shut down theirCDMA network, therefore requiring devices to supportLTE or5G. Customers withCDMA-only devices andLTE devices without VoLTE support would have been required to switch to a VoLTE-capable device by that date. Verizon stopped activating CDMA-only devices on their network in 2018, and had previously planned to shut down 3G service in 2019, but extended the timeline for shutting down the 3G network twice[47] — first to December 31, 2020, and then to December 31, 2022, which the VP of Network Engineering says "will not be extended again." The company indicated the delays were in an effort to "minimize disruptions" to its customers still utilizing the 3G network. As of March 2021, less than 1% of Verizon's customers were still using 3G, and many of the 3G-connected devices are integrated devices, such as smart utility meters and home burglar alarms.[48]
Additionally, certain Verizon-compatible handsets were blocked from Verizon's CDMA network even before December 31, 2022, even if the device supported CDMA (for instance, unlocked devices with support forSprint orChina Telecom), therefore requiring VoLTE support when used on Verizon. This includes5G-capable andOnePlus handsets. Devices lacking CDMA support starting with thePixel 6 andiPhone 14 series as well as select older devices sold on GSM carriers will only work on Verizon in VoLTE mode.
To ensure compatibility,3GPP demand at leastAMR-NB codec (narrow band), but the recommended speech codec for VoLTE isAdaptive Multi-Rate Wideband (AMR-WB), also known under the trademark HD Voice after GSMA's certification program. This codec is mandated in 3GPP networks that are capable of 16 kHz sampling.[49]
In addition, many carriers and devices can useEnhanced Voice Services (EVS). This is an up to superwideband (20–16,000 Hz) or fullband (20–20,000 Hz) codec backwards-compatible with AMR-WB.[50] This codec is also known under the trademark HD Voice+, after GSMA's certification program.[51] GSMA has proposed to make EVS mandatory just like AMR-WB.[50] (Both the carrier(s), any interconnect and the two calling devices must be capable of using the codec for it to be used.) The AMR-WB+ codec has a wide bit-rate range, from 5.2 to 48 kbit/s. EVS offers a wide range of bit rates from 5.9 kbit/s to 128 kbit/s, allowing service providers tooptimize network capacity and call quality as desired for their service, so VoLTE does not ensure high call quality.[50]
Fraunhofer IIS has previously demonstrated an implementation of theAAC-ELD codec in VoLTE that they call "Full-HD Voice". It has not gained any standard status or real-world adoption.[52] They have since reused the term "Full-HD Voice" for EVS in fullband mode (HD+ is also used).[50]
Cellular network standards | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| 0Gradio telephones (1946) | |||||||||
| 1G (1979) |
| ||||||||
| 2G (1991) |
| ||||||||
| 2G transitional (2.5G, 2.75G, 2.9G) |
| ||||||||
| 3G (1998) IMT-2000 (2001) |
| ||||||||
| 3G transitional (3.5G, 3.75G, 3.9G) |
| ||||||||
| 4G (2009) IMT Advanced (2013) |
| ||||||||
| 5G (2018) IMT-2020 (2021) |
| ||||||||
| Related articles | |||||||||
