T-Mobile has had VoLTE (Voice over LTE) for quite some time, and they are now looking to improve their VoLTE call service for their customers and are said to be quietly testing out Enhanced Voice Services on the LG G5 - which launched last Friday at T-Mobile. For those that might be unaware of what Enhanced Voice Services or EVS is, it's a technology that is at the heart of Full HD Voice. It's a next-generation technology. So think of 4G and how it was a fourth generation of mobile data, replacing 3G. EVS is here to replace the HD voice codec AMR-WB, which is what most carriers are currently using. What happens with Enhanced Voice Services here is that it uses complex speech compression algorithms, additionally, it's made to provide even higher-quality audio than what cellular networks use today. According to the 3GPP who standardized EVS, it will allow HD voice services to gain "even crisper and more natural voice quality in any kind of communication environment."
There is a thread over on Reddit that shows a screenshot of the engineering settings on the brand new LG G5 for T-Mobile. This screenshot shows some support for EVS, which leads most people to believe that T-Mobile is working on EVS with the LG G5. At least getting started with it. It'll be available for other phones once it's launched by the carrier.
T-Mobile has been working diligently on improving their network. When Legere first took over T-Mobile back in late 2012, he stated that his company's network was not a great one. But now the network has improved greatly, and added a ton more coverage as well. But it looks like Legere and his team is far from finished here, and is looking to improve their network even more. They have been pursuing HD Voice and VoLTE technologies quite a bit lately, so it's no surprise that they are working on Enhanced Voice Services. It's something that most end-users won't even notice, but it'll be a great addition to T-Mobile's network for making calls. Soon, the voice network that we all know and have used for years, will cease to exist, with carriers jumping on Voice over LTE or VoIP calling.