Sony demos VoLTE – telecom-grade VoIP over LTE [video]

A few weeks ago, we highlighted LTE coverage for Xperia smartphones to let you know which devices had LTE support and the regions which generally offered LTE service. Now, we’d like to share with you the next step in the evolution of LTE – making voice calls over LTE, which we’re currently demonstrating at Mobile World Congress. Voice over LTE (VoLTE) gives you telecom-grade voice functionality over IP while keeping the high speed data connection. Read on as Sony connectivity engineers, Daniel Lönnblad and Pär Olsson, share their VoLTE expertise.

Daniel Lönnblad, Research Engineer at Sony.

Pär Olsson, Engineering Manager at Sony.

Hi, we’re Daniel Lönnblad and Pär Olsson, connectivity engineers at Sony, and now is a great time to start talking about the VoLTE, because this week’s Mobile World Congress where we’re cooperating with a number of other vendors to demonstrate VoLTE. At MWC, we are demoing development software with VoLTE functionality running on modified Xperia™ V prototypes in multiple stands – Ericsson, Telefónica, Qualcomm, and others. But first, let’s explore the background of LTE and Voice over LTE.

Background to VoLTE
LTE, or Long Term Evolution, provides higher data transfer rate and capacity and lower latency in the mobile environment through new modulation and all IP network. LTE is defined in global 3GPP standards. As interest in LTE continues to grow, and more operators are starting to carry the services over LTE, we at Sony are already on board with the technology, as you’ll find a number of LTE-supported Xperia™ smartphones available on the market.

The way to make voice calls in today’s network is to move the user back to 3G or 2G when a phone call is made or received, this is called circuit switched fallback (CSFB). The ability to make telecom-grade voice calls over LTE is accomplished by means of an IP Multimedia Subsystem (IMS) network specified in 3GPP. Several operators and vendors have profiled the final solution GSMA VoLTE IR.92 specification, which uses a horisontal control layer that isolates the access network from the service layer to aid in the access of voice applications between devices. This removes the dependency of the legacy circuit-switch network and will long term allow operators to only have packet-switch IP network.

LTE benefits
With LTE, end users benefit from improved data transfer and communications. Operators also benefit from LTE by optimising this network infrastructure to provide high-capacity and high-speed data communications. When operators deploy LTE, they can enhance the voice call experience with HD voice and beyond, also more convenient way to connect services like chat, video and file transfer to the voice call. This gives users superior voice quality during calls and seamless using additional communication methods while talking.

VoLTE vs Skype & Google Talk
Given the benefits of VoLTE – broad bandwidth, low latency, and IP-based – the technology is clearly an evolution for mobile broadband voice services. Similarly, another approach based on the usage of over-the-top (OTT) voice services can also be provided over an LTE network. Examples of OTT voice include Skype and Google Talk, applications that enable voice calling over IP for data tariffs. However, VoLTE is still a better solution compared to OTT voice for several reasons.

First, VoLTE can be transparent to the end users, whereas in an OTT VoIP solution, both sides must use the same application to communicate. With VoLTE, everything is integrated in the default phone app and the network handles the interoperability. It is also the same out of the box experience as voice call today with no configuration effort.

Next, the evolution of VoLTE supports the possibility to upgrade an incoming VoLTE voice call to a video call, or downgrade an incoming video call to a VoLTE audio call. Quality of Service (QoS) is another important differentiator between the two. Since VoLTE is an operator service, it will run with a priority over other best-effort data traffic, always giving the best possible user experience. VoLTE also supports handover to legacy (3G/2G) network without interrupting the phone call when moving out of LTE coverage, defined as single radio voice call continuity (SRVCC).

VoLTE – changing the game in telecom
VoLTE is the next step for voice, as indicated by the growth of LTE networks. But what makes VoLTE such a compelling offering?

It starts with the fact that VoLTE operates as a native application in the device, enabling prioritisation over other data streams to have the ability to deliver uncompromised voice quality even beyond toadies HD voice. Users may need to embrace a new way of calling since VoLTE enables super fast calling and will ring instantly at the other end. For example, at Sony, we’ve been able to achieve a one second call setup time in a live network setup.

The evolution of VoLTE also provides enhanced native voice experiences such as video telephony, which is all IP-based, and gives the possibility to switch between voice and video service in an ongoing call. For operators, VoLTE allows for more competitive offerings, the potential for increased network capacity, and in areas where there is limited or no 2G or 3G voice coverage, operators who have LTE 700-900MHz licenses can still offer in-network voice service.

Finally, as VoLTE evolves and network APIs become available, developers will be able to create their own services without having to develop their own messaging or calling technologies, they can just utilise the services and inter-connectibility provided by LTE networks.

Telefónica first to demo VoLTE handover
One of the pioneers in VoLTE making great progress is Telefónica, a leading telecom operator in Europe and Latin America. They recently demonstrated the first, successful test of transferring voice calls seamless from an LTE network to a 3G mobile network at a lab in Germany.

Last summer, Telefónica Deutschland showed how they did the handover of LTE calls to 3G through Single Radio Voice Call Continuity (SRVCC). This is an LTE functionality that allows a VoIP and IMS call in the LTE packet domain to be moved to a legacy voice domain without breaking up. This pioneering breakthrough paves the way for more efficient networks in the future, along with media-rich commercial services.

And as mentioned in the beginning of this post, at MWC this week, Telefónica is demoing pure VoLTE voice services (LTE to LTE calls) using commercially available Xperia™ V smartphones with modified chipsets and modified software support for VoLTE.

Challenges of implementing VoLTE
Many mobile operators are expanding and migrating mobile networks to LTE in order to manage the data traffic explosion. Within the ecosystem, there are some obvious challenges to delivering the Voice service over LTE networks. From a commercial standpoint, operators must invest in moving the voice service to LTE and ensure it will have the same high service grade as legacy networks.

On a technical level, theVoLTE infrastructure is quite complex, and involves different network and IMS (IP Multimedia Subsystem) vendors, operators, device manufacturers. Extra effort needs to be added to make all handset vendors interoperable since most of the signalling when setting up services are made between handset and not only between handset and mobile network as previously.

But in time, best practices should arise from lessons learned by early adopters, and will make it easier for new implementations, but for now, these challenges exist for VoLTE.


We hope this gives you some valuable insight into voice over LTE. If you’re at MWC, come check out the VoLTE demos at our partner booths (please note that these are strict research demos, not commercial products). And feel free to leave us any comments or questions about VoLTE below, and we’ll get back to you as soon as we can.

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  1. By Marcel Endris


    What about the Xperia Z2 D6503? The contract and network allows it, just not the phone

  2. By Mousa Khalid


    Hello Joe,

    Thank you for being cooperative with the comments,

    I would like to double check something about VoLTE support:

    Is all Sony Xperia Z3, Z4 & Z5 HW can support VoLTE and just need firmware customization per operator, or there is different HW versions per region?? here I am talking about MEA region.

    In case some operator planned to launch VoLTE, they can consider all their users with Xperia Z3 and above as potential VoLTE users or not necessarily?


  3. By Nishant Badadali



    This is a wonderful forum with great responses!!!

    I in one of the mobile operator showing demo for the VoLTE to VoLTE call… We have our network setup with huawei eNODEB + EPC and IMS. I am able to successfully attach with EPC for my internet bearer and connect to 4G LTE.

    But for VoLTE registration to IMS, I believe my Sony Z2 mobile dose not have the software build or SIP stack.

    Can you suggest any software’s or build I can put it in my mobile an do successful registration.

    Thanks !!!

    • By Joe Padre


      Hi Nishant,
      Thanks for your feedback. Could you contact us directly: (replace _at_ with @), so that we can establish a formal contact for you?
      Thanks and best regards,
      Joe from Developer World

  4. By Dohoo Kim


    So, is there’s any plan to support generic VoLTE configuration for international XPERIAs? As time passed, the VoLTE-related IMS / 3GPP standard is completed and not draft-level anymore. Some carriers, like LG Uplus of South Korea, already switches their 2/3G network to LTE-only /w VoLTE.

    • By Joe Padre


      Hi Dohoo,
      Thanks for your question. As you’ve pointed out, it’s the carriers/operators who plan launches of VoLTE service for their networks, not the handset manufacturers. And although there are VoLTE standards, each carrier can decide upon their own configuration of their VoLTE-enabled Narrowband and Wideband (HD Voice) channels.
      Sony is very active in ensuring VoLTE functionality towards as many operators as possible, and we are also supporting initiatives to narrow down the differences between VoLTE networks across the world. We have a pretty good picture of what a generic VoLTE configurations consists of, and we’re continuously supporting carriers with devices for testing, prior to launching. However, we still see a need to test toward every specific operator to ensure the right functionality and user experience, and when this can be done, we will consider making VoLTE more generally available. For now, Sony Mobile handsets from Xperia Z2 and onwards are VoLTE-capable.
      Best regards,
      Joe from Developer World

  5. By Jameel M


    Hi. We are a Telecom Subcontractor who are testing VOLTE for our vendor client. Can you please suggest how we can get the software update for Xperia Z1 to be able to test VOLTE feature on the mobile?

    • By Joe Padre


      Hi Jameel,
      Thanks for reaching out to us. Unfortunately, we don’t have any “generic” VoLTE configuration which we could enable in SW and send out for testing. Instead, we encourage you to ask your vendor client to approach Sony Mobile officially (have them send an email to at
      Best regards,
      Joe from Developer World

  6. By Ralf Patzwaldt


    Hi, 4 weeks ago I bought a brand new Xperia Z2. Is this handset VoLTE ready? or will be ready with a software update? Network: O2 Germany

  7. By James Rosenberg


    I use U+ LTE in korea (LTE Bands 850Mhz and 2100Mhz with 2600Mhz coming soon) my current phone uses VoLTE service by default and will roll over to 3G (CDMA) if I ever am outside of an LTE area. Seeing as how my carrier is going to be exclusive LTE and 3G CDMA is already being phased out I am curious about one thing. My carrier offers VoLTE, It offers it in the 850Mhz band that supposedly works with Sony phones in the demo, but before I drop $800+ on a Z Ultra or Z1 I would like to know …. can I make calls via VOLTE, or do I need custom software on the phone (from my carrier) to make it work? mjrii87 AT gmail . com for direct message

    • By Joe Padre


      Hi James,
      Though your carrier offers currently VoLTE, alignment must first be done jointly between the carrier and us, and some of configuration that may be required cannot be pushed to the device through a software update. So, you would not be able to make calls via VoLTE on Z Ultra or Z1 with a custom software update alone.
      Best regards,
      Joe from Developer World

    • By Joe Padre


      Hi George,
      I checked with Pär on this, and he confirms that no, Xperia V doesn’t support VoLTE in the latest release. He also says that VoLTE as a feature, needs to be rolled out hand in hand with carriers enabling it in their networks. In the meantime, you may want to check with your carrier to ask when they will enable VoLTE in their network.
      Best regards,
      Joe from Developer

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