For the launch of the iPhone 6S, I elected to buy the T-Mobile version from Apple instead of the AT&T one, because I was planning on paying full price for the phone and the T-Mobile one comes unlocked out of the box. (I don't plan on leaving AT&T, necessarily, but an unlocked phone is easier to use internationally and has a higher resale value.)
I suspect there are some folks out there who did the same. What you may not have known, though, is that using this version of the phone on AT&T sets off a weird chain of events that you'll want to be aware of.
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. (PC Magazine's Sascha Segan does a good job breaking down what that means here.) 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.
Bear with me, there's a lot going on here
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. Some iPhone users (myself included) are seeing this message when they try to enable it through the Settings menu:
The reason, most likely, is that your account isn't set up for VoLTE.
But how did that happen? It's my understanding that AT&T certifies phones for Wi-Fi calling (and, by association, VoLTE) on a per-SKU basis; the AT&T models were obviously certified on day one, but the unlocked T-Mobile versions are not as of yet. They're going through certification at present, though — and barring any hiccups, they should be ready soon. I believe that once that happens, you'll be able to enable Wi-Fi calling without issue.
In the meantime, you can call in to AT&T's customer service line if you like, but in my experience, they aren't well-prepared to deal with this error, and they don't understand what causes it when you call in. To make matters worse, the customer service team can't turn on VoLTE on your account themselves if you're using an uncertified device — it requires that a special trouble ticket be filed. As of yesterday when I spoke to AT&T's tech team, the backlog for dealing with these trouble tickets was 11 days, I was told. If you're using a certified phone like the WCS-enabled model of the 6S, though, it's not a problem at all. "Any care rep can correct VoLTE provisioning as long as the user has a capable SIM and AT&T certified device," an AT&T spokesperson tells me.
If you haven't yet purchased your iPhone 6S, the good news is that Apple is now selling unlocked iPhones that support WCS. I'm hearing that these new SIM-free models should simply work out of the box, unlike the T-Mobile ones.
Again: none of this should affect you if you bought your iPhone directly from AT&T, or you bought the AT&T model from Apple. Otherwise? It's a little complicated.