What cell phone companies are GSM?

The GSM Association maintains a list of phone companies operating GSM networks:

http://www.gsmworld.com/roaming/gsminfo/cou_us.shtml

AT&T and T-Mobile are on the list, Sprint and Nextel are not. Generally speaking, American phone companies lag the rest of the world in adoption of GSM, having invested a lot into CDMA infrastructure in the early days of cellular telephony…

As to unlocking the phone, yes, you can use it with any company that supports the phone’s communication protocol (CDMA-only phone won’t work on a GSM-only network and vice versa). You may or may not be able to use services other than basic telephony though, as those often require a special chip and/or software that phone companies ask phone manufacturers to install.

GSM network operators: GSM ( Global system for mobile communications) GSM uses TDMA (Time Division Multiple Access) technique in mobile phones . The use of the SIM card allows GSM users to quickly move their phone number from one GSM phone to another by simply moving the SIM card. GSM users can also switch over from one network to another network by unlocking their mobile device. Nowadays, many of the online vendors like http://www.unlock-free.com/ generates free unlock code to unlock a phone from network lock . Yes ! if i buy a new phone and unlock it from sim lock , i can use my phone with any other GSM company . Use the link http://phone-solutions.pavemyway.com/Gsm-Operators/Gsm-Operators-List.php to know about GSM mobile operators in the world .