This is from the Textra message board.
T-Mobile MMS over WiFi Calling – Textra SMS
T-Mobile MMS over WiFi Calling
← Knowledge Base
Thanks for the feedback.
Regrettably, T-Mobile does not grant any third-party messaging apps access to the T-Mobile WiFi Calling feature for MMS. T-Mobile use something called GBA authentication which only system apps or apps signed by t-mobile can use (so not Textra).
We are working with t-mobile to hopefully allow us access!
In the meantime ensure in Textra, Settings, MMS Settings, Prefer WiFi is off.
Thanks.