How/Why is AT&T phone locked to T Mobile service?

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I bought this S7 phone used over a year ago. The IMEI is not blacklisted. It APPEARS to be an AT&T phone, it boots up with the AT&T loading screen, and has the traditional AT&T bloatware/features.

When I went to activate the phone on Straight Talk, it didn't work using the included AT&T sim card, I even returned the BYOP kit thinking maybe it was a bad sim card or something. When the second one didn't work I tried the T Mobile sim card just for the hell of it, which DID activate the phone.

So it seems like this AT&T phone is locked to T Mobile... I tried AT&T's automated unlocking process and they tell me "Looks like the IMEI number you entered doesn’t match an AT&T device." The T Mobile unlocking process seems like it would be built into the phone itself as an app. However, it's an AT&T phone so I don't even have that app. So I called T Mobile and they pretty much claim they cant even look up the device which is locked to their service because the phone is no longer in service with them. They told me to "contact the manufacturer"...which technically would be Samsung? LUL what?

So from what I'm getting out of it is... If you bring an unlocked phone to the T Mobile network and then end your contract, or even just take the phone off the contract, they keep the phone locked to their service for life for absolutely no reason? lol it's almost as if that's sketchy. I can't imagine how that is even legal for them to be able to lock another company's device to their service without even having any record of it. The only reason I can think of would be that the lady who sold me the phone owed T Mobile for a bill that was never paid, which she assured me she didn't owe them anything. However they are clearly unable to even look up the device at all, much less tell me if this "device" owes them money or not lol.
EVEN IF the people didn't pay their bill which I don't believe is the case, that will go to collections or whatever they choose to do, how do they have the right to lock another company's phone to their service on top of it? That would be like your internet service provider locking your Dell PC to only connect to the internet through their service, then when you cancel their service, it's locked to them forever for no reason. oh and by the way they don't even know how to look your computer up to unlock it, much less to provide a reason for it being locked in the first place.
That sounds like an incredible deal.

Does anyone have any ideas or insight on this situation? I would prefer to use an AT&T sim card/service. If no ideas, am I the only one who sees this as a pathetic business practice? or is this standard procedure with all cell companies?

B. Diddy

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Mar 9, 2012
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Welcome to Android Central! I recently read how this happens with iPhones (as part of Apple's reseller flex policy) -- there are many threads out there about this, like this one: I recall seeing one thread about this recently for Android devices, but it doesn't seem to be a common complaint.

It may be a pain, but maybe you could purchase a prepaid SIM from T-Mobile for a month, and while you have it on T-Mobile for that month, ask T-Mobile support to carrier unlock the phone at that time.