Written by 
“You can use any handset on our network you want,” says Ralph de la Vega, CEO of AT&T’s wireless business. “We don’t prohibit it, or even police it.”
AT&T has started promoting this open network policy as if it were something new. In reality, this is the way it has always been.
AT&T devices operate on a GSM network. This is the most widely used network technology in the world. Your phone number and related data is stored on a subscriber identity module(SIM) card. This SIM card can be placed into any other GSM compatible cell phone. Unlocked T-Mobile phones will work fine, but Verizon Wireless phones which operate on a CDMA network, are incompatible with AT&T’s GSM network.
So, this was really a marketing and public relations exercise, and not a dramatic change in policy.
