Any non-Branded to T-Mo phone and you give up WiFi Calling. Period. No reprieve. No workaround. Also if T-Mo gets the SoftCard tap-to-pay App, you won't get it on a phone not branded T-Mo.
The 1520.3 does get every data speed on T-Mo up to and including LTE. I have one. I was on T-Mo. Now I'm on Straight Talk with an AT&T style SIM. Works great there too.
The RM-938 1520.3 has the following advantage over the AT&T RM-940. The 1520.3 has Qi Charging and AT&T does not. Every 1520.3 has 32GB internal storage standard. AT&T has a rare upgrade to 32GB all others being 16GB and every AT&T 1520 needs a special case to charge wirelessly on the PowerMat format.
However, an unlocked AT&T model WILL run every data speed on T-Mo. There is a trick to unlock the valuable 1700 MHz on the 3G radio and all is golden. Same as 1520.3 without the trick.
http://forums.windowscentral.com/nok...-4-1700-a.html
B&H Photo seems to be the preferred seller for 1520.3 in the US according to posts on this forum. It is where I got mine and their price is usually lowest or lower than most for Brand-New-In-Box. They are a reputable seller with a solid reputation.
Nokia Lumia 1520 RM-938 32GB Smartphone A00016719 B&H Photo They currently have white and yellow in stock... Guess which one I have.
BTW - Last August I had reached the same decision. 1520 was the only true upgrade from a 925. I've not been sorry 1 second I traded up.