I thought that only the T-Mobile branded 640 had WiFi calling that worked on T-Mobile.My understanding is that it will be just fine. At one point, there was an app for it which did not work well for us. In the Lumia 640 it is built in and works great.
Has anyone bought an unlocked 950 and tried to see if it supports tmobile wifi calling? Since 8.1 update 2 it was baked into the OS.