I've looked at issues other users have had with this phone and mine are similar. However - there weren't any solutions.
I believe the root cause is the Wifi calling. Is there a way to turn it off so I use the TMobile network only? I do not receive ANY calls when it's in Wifi mode. On some occasions, I can dial out - but those calls have all disconnected abruptly during about 5 minutes into the call. The other calls - it says it's dialing, but just hangs. You can NOT disconnect the call unless you turn off the phone.