How to activate wifi calling on unlocked s6 Active?

[AC Question]

Last year I bought an s6 Active from AT&T, who gave me the unlock code to use with Tmobile. There was no wifi calling option on the phone when I purchased it.

I just got my s6 active back from being repaired at Samsung. The packing slip says they repaired screen and installed some new parts, but it now has features that were not previously available, including AT&T wifi calling. However, when I tried to activate the wifi calling feature, it did not work, giving me some AT&T fail type message.

Obviously the phone has the capability for wifi calling, so how do I enable it? I have an unlocked s7 that works fine on wifi calling, and guessing it's the same software. So is there some type of unlocking I can do to enable it on the s6?

Thanks