Sony Mobile is working with operators to add VoLTE (voice over LTE) support for the Android Nougat Concept firmware for the Xperia X. The benefit of VoLTE is that calls should connect faster and have better sound quality too, there is no need to switch to a 2G/3G frequency to make a call.
However, it is somewhat difficult to implement the technology as it requires both phone and network support. This means that Sony will need to add support on an operator by operator basis. The next Concept update will include VoLTE support for three of the largest German mobile carriers including Deutsche Telekom, Vodafone and Telefonica/02.
Sony will then look to add UK and Spain after this. You may be interested to hear, that the three largest markets for the Concept release is Germany, UK and Scandinavia. This is perhaps the reason to focus on the German and UK markets for VoLTE first. We will keep you updated once the new VoLTE firmware starts to roll.