Is it possible to switch carrier firmware on the Galaxy S5?

[AC Question]

I was given a AT&T Galaxy S5 by Samsung and they unlocked it. I've used it internationally on T-Mobile and now want to use the wifi calling feature, plane text, and other features that T-Mobile offers and AT&T doesn't but since the phone has AT&T firmware they're not available. Is there a way to switch the firmware or enable them?

 

[Rukbat]

Not only is the firmware missing those functions, the hardware is too, so no. (And the bootloader is locked, so that would be another obstacle.)

 

[Golfdriver97]

Welcome to the forums.
To add to what Rukbat said, if you flash a ROM from a different carrier to that device, there will be a very high probability of bricking it.