How to convert my iphone from tmobile to at&t?

I purchased an iphone 5 from a charity org. And the phone was locked on the icloud screen. Got past that but it will not accept my sim card, it says my sim card is not supported under the activation policy. Can I take the phone to AT&T and have them convert the phone over to them?

It may be completely locked down. If it was ever listed as stolen (even accidentally) it would be entered into a database that prevents it from ever being used again on any wireless carrier. I wold call T-Mobile and have them check, if that is your carrier, they should be able to check that data base to see if it is eligible to be used at all. If it is locked i would return it to the charity you bought it from.

It's still locked from AT&T. You just need to contact AT&T and get the unlock code from them. As long as you own the phone and are not in a contract, they are required by federal law to provide you with a way to unlock the phone so that it will accept other carriers SIM cards. The only thing that can prevent you from using it is if it has been reported stolen, in which case the IMEI of the phone has been blocked and there's nothing you can do.

AT&T will not help you with the unlock. You need to get the phone factory unlocked from outside your carrier. If you need to get the phone factory unlocked, mail me for references.