For all the boasting about WiFi calling that T-mobile is doing, it sure doesn't work very reliably yet. I had the same issue for the brief time I was with T-mobile and tried using their WiFi calling. Often times my phone 5s wouldn't even call out, it was attempt the call and then fail right away. Usually if this happened, it would continue to happen throughout the day. The next day, WiFi calling would work perfectly on the same WiFi network. This leads me to believe the issue is on T-mobile's end. I tried all of the suggestions from people, opening multiple ports on the router(s), and changing sim cards. Nothing worked reliably. Except switching to Verizon and not having to use WiFI calling in the first place
