This was posted on Tmobile blog:
Tmobile is turning on VoLTE in Seattle. I will try to explain this as best as I can since I work in the Industry and atm Tmobile. You can read further techs in the blog post. But simply, Tmobile is putting its Voice network on its LTE network.
As you know, our LTE devices, operate on all (US) GSM/CDMA/LTE Bands but voice calling we were always locked out of (because of Google). You can see if you are using a regular smartphone, if you place a call, your phone will swtich from LTE to 4G service. The current standard. But with Tmobile and ATT rolling VoLTE out, our phones will stay on LTE provided you are in the service area.
Inside these cabinets there used to be two devices that handled each technology, GSM and LTE. But i just started seeing Tmobile roll out a new devices that carries both of these technology in the same unit, providing this capability. I can tell you NYC and the Boroughs roll out is wwwwaaaaaaaayyyyyyyy behind. We have completed 1500+ LTE upgrades around the city and i just started seeing the new device about 3 weeks ago. So i can tell you that Tmobile has alot of sites to go back and retrofit in order to make this work in NYC. ATT will be done in NYC before tmobile, because they are getting ready to upgrade their LTE network but they will install the newer device at the same time. ATT should be ready by years end. Tmobile, will probably turn it on by end of year as well but the service will be very scattered.
Now i wonder if some devs with deeper knowledge of these devices and programming could turn these devices into a phone. As of right now, only a developer in Seattle could work on this (For Tmobile). And where ATT has turned on this service. Illinois, Indiana, Minnesota, and Wisconsin. So maybe there could be hope on the LTE devices. God speed developers.