AT&T throttled data speeds on an unlocked phone I had a couple of years ago and when I called they said “we can’t guarantee that your service will work properly unless you buy an AT&T phone.” They never restored service back to normal after they found out what kind of phone I had. This was on a two year contract, not even prepaid. I’m so glad I switched to T-mobile, because AT&T has absolutely no problem picking and choosing how they go about being evil.