GSM ,AT&T network technology used by more than 80 percent of the phones in the world wide.The GSM phones store all the data info on a SIM card ( subscriber identity module ) so you can remove the data card (SIM) and place it into another unlocked cell phone.in any network you like as long as it has GSM technology.

Got an T-mobile phone you want to use with AT T? As long as they are a GSM-compatible phone, and you have really unlocked it,it will work.
You can use your unlocked phone in England,Canada,or yes even China.

ATT has always has had this open,unlocked policy,but just never has made it known to the public like it is doing now.In fact ATT is the one carrier that is willing to be open ,er unlocked that is) about their cell services and unlock cellphones.

Thats why they will be the front runner in unlocked cell services

Used at&t 3G SIM Card GSM cell phone att iPhone unlock    US $3.49

Apple iphone 4 - 32GB Black Unlock for AT&T-Sealed    US $670.00

ATT AT&T SIM Card Unlock iPhone 3GS GSM Cell Phone GSM    US $4.75

Apple iphone 4 - 32GB Black Unlock for AT&T-Sealed    US $610.00

Apple iPhone 3GS 16GB Black AT&T Easy 2 Unlock req35376    US $410.00

ATT AT&T SIM card Unlock iPhone 3GS GSM Cell Phone GSM    US $5.00

New Apple I Phone 4 16GB Black, ATT ,could be unlock    US $799.00

Apple iPhone 3GS 16 GB Black Unlock (AT&T)    US $550.00

ATT AT&T SIM card for Unlock iPhone 4 GSM Cell Phone    US $9.75

3G Sim Card for ATT/AT&T IPhone 3g 3gs GSM Unlock    US $3.99

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From: Unlocked cell phones

AT T how they see unlocked phones