I'm in a bit of a predicament regarding purchasing my two S6 Edges. I've been on my two S4s for two years and got a Best buy protection plan for them which ends next month. Lucky for me the plan junked out and I'm getting two Bestbuy gift cards for the original value of my phones. The thing that sucks is they don't sell T-Mobile phones, or at least the T-Mobile S6 edge. I was leaning towards getting two At&t versions of the phone and unlocking them. However, I am not sure if features like VoLte(Voice over LTE). If I'm not mistaken LTE should work but I'm not to familiar with the At&t Edges frequency bands and Tmobiles network bands. Any help or suggestions would be greatly appreciated.
See attachments for bands supported by both AT&T and T-Mobile variants. If you're asking if both variant support VoLTE, they do.
The AT&T variant appears to covers all T-Mobile LTE bands but does not seem to cover the 1700 band.
Check threads on previous AT&T Galaxy models on T-Mobile.
I wish I could give you better clarity on using an AT&T device on T-Mobiles network, but I just haven't done it. I've only done it the other way around. I can tell you that I have done this for several generations of Galaxy releases and the results in performance has only been subpar on one occasion. I think that was the Note 2 or Note 3. I don't remember which one right now, but the data was just a little slower at times. This isn't what you're trying to do anyways.
I hope that help a little.