The WiFi calling on Tmobile phones is something Tmobile adds. It uses your minutes, just allows you to talk over WiFi instead of a cellular connection.
Using Google Voice and GrooveIP
always works great for me. You will just have to use your Google Voice number instead of your Tmobile number, but that's it.
"Is life so dear, or peace so sweet, as to be purchased at the price of chains and slavery? Forbid it, Almighty God! I know not what course others may take; but as for me, give me liberty or give me death!"
-Patrick Henry, March 23, 1775