It looks like there might be more to the story about the AT&T-carried 950 family than meets the eye. Consider the article from The Verge from 10/9 entitled, "Can't use AT&T's Wi-Fi calling with your iPhone? This could be why".
First off, there's the consideration that AT&T has a special version of the 6S that includes support for the WCS band, which will eventually lead to faster LTE in some markets. When you buy the T-Mobile version and use it on AT&T, you won't be able to take advantage of WCS. This is a relatively minor consideration, because WCS is just now rolling out and won't have a dramatic impact; it's something to think about, though.
But when you drop an AT&T SIM into the T-Mobile iPhone, some unpleasant things happen on the back end. AT&T's systems appear to automatically de-register you from Voice over LTE (VoLTE), which means you will drop to 4G whenever you make or receive a call and you won't be able to use HD Voice. AT&T is aggressively expanding its VoLTE footprint right now, so it's arguably more important than the WCS issue.
And here's something else: AT&T's systems tie VoLTE and Wi-Fi calling together, which means that if your account isn't registered for VoLTE service, you won't be able to use Wi-Fi calling, either.
AT&T is almost as bad as Verizon.
Consider this before buying a new unlocked 950/XL from the MS store to work on AT&T. You might be caught up in something like this.
Those of us on T-Mobile are in a better spot.
First off, there's the consideration that AT&T has a special version of the 6S that includes support for the WCS band, which will eventually lead to faster LTE in some markets. When you buy the T-Mobile version and use it on AT&T, you won't be able to take advantage of WCS. This is a relatively minor consideration, because WCS is just now rolling out and won't have a dramatic impact; it's something to think about, though.
But when you drop an AT&T SIM into the T-Mobile iPhone, some unpleasant things happen on the back end. AT&T's systems appear to automatically de-register you from Voice over LTE (VoLTE), which means you will drop to 4G whenever you make or receive a call and you won't be able to use HD Voice. AT&T is aggressively expanding its VoLTE footprint right now, so it's arguably more important than the WCS issue.
And here's something else: AT&T's systems tie VoLTE and Wi-Fi calling together, which means that if your account isn't registered for VoLTE service, you won't be able to use Wi-Fi calling, either.
AT&T is almost as bad as Verizon.
Consider this before buying a new unlocked 950/XL from the MS store to work on AT&T. You might be caught up in something like this.
Those of us on T-Mobile are in a better spot.