I've had a lot of problems with an unlocked AT&T "Samsung Galaxy S7 Active" trying to get it to work on T-Mobile.
This was a phone I bought myself, and I got it unlocked myself (and had it on Cricket division of AT&T for most of a year)
It's been quite a struggle. I've had several T-Mobile people on voice working on this.
What is it doing now, or last time I checked?
- Voice and text messaging frequently die. This requires me to turn Airplane mode on, and then off again. This almost always fixes this problem. Until it does it again. Needless to say, this makes the phone useless for receiving urgent phone calls and texts, because chances are the connection will have died later when one of these calls or text comes in.
- There is NEVER any connection to LTE! The best I get is T-Mobile HSPA or AT&T roaming. Even when I verify that the T-Mobile bands are accessible, and also verify that it works great from another phone. I can't get to Band 12 even when I force the phone using *#2263# to only use LTE Band 12.
- When roaming into AT&T, it is often "emergency calls only".
What has been done?
- The Samsung dial-pad radio check of *#2263* has been toggled various ways with some improvement (WDCMA only seems to help sometimes), but it is usually left at the default: automatic with everything on.
- The proper APN, T-Mobile LTE US, has been created and destroyed and created with guidance from T-Mobile help many times.
- This APN has been tried, as has the default T-Mobile GPRS APN.
The T-Mobile SIM is good: I've removed it from the S7 Active and put into an unlocked T-Mobile supporting "Axon 7".
- When in the "Axon 7", I get (relatively) blistering speeds of from 5 to 9 mbps on LTE, usually Band 12 when I check.
- Roaming onto AT&T works better.
- I've never had "emergency calls only" when in AT&T territory.
- I can't recall seeing voice/text EVER die...