activate att iphone 4 on tmobile

If the phone came from AT&T, they are the only ones that can unlock it. However, AT&T never unlocks iPhones and have not done it since they started carrying the original iPhone in 2007. Your only choice is to sell the phone to someone and either purchase an unlocked iPhone direct from Apple, or if you do not want to pay the full price to buy one like that, you may see if you can find an unlocked phone on Craig's List or eBay. There is no supported way for you to unlock the phone. The tech was right when it said AT&T would have to be the one to unlock it. They have to send the authorization to unlock to Apple, but they won't. Sorry.

If the phone came from AT&T, they are the only ones that can unlock it. However, AT&T never unlocks iPhones and have not done it since they started carrying the original iPhone in 2007. Your only choice is to sell the phone to someone and either purchase an unlocked iPhone direct from Apple, or if you do not want to pay the full price to buy one like that, you may see if you can find an unlocked phone on Craig's List or eBay. There is no supported way for you to unlock the phone. The tech was right when it said AT&T would have to be the one to unlock it. They have to send the authorization to unlock to Apple, but they won't. Sorry.

The only way to use an iPhone with T-Mobile is to buy a factory unlocked (full price) phone directly from Apple (on line or physical store).  Any phone that was used with AT&T is guaranteed to be locked to it. 

Don't trust phones that are hacked to unlock - they are unstable and will re-lock to AT&T whenever updated or restored (sometimes randomly).

Don't trust (unlocked) phones bought on ebay or craigs list - 99% are locked

NOTE:  Even if you do get an officially unlocked phone that works with T-Mobile USA, you won't get 3G service, only the much slower Edge (2.75G).  T-Moble uses nonstandard 3G frequencies that are not compatible with iPhones.

The only way to use an iPhone with T-Mobile is to buy a factory unlocked (full price) phone directly from Apple (on line or physical store).  Any phone that was used with AT&T is guaranteed to be locked to it. 

Don't trust phones that are hacked to unlock - they are unstable and will re-lock to AT&T whenever updated or restored (sometimes randomly).

Don't trust (unlocked) phones bought on ebay or craigs list - 99% are locked

NOTE:  Even if you do get an officially unlocked phone that works with T-Mobile USA, you won't get 3G service, only the much slower Edge (2.75G).  T-Moble uses nonstandard 3G frequencies that are not compatible with iPhones.