If you have an unlocked phone or smart phone, then you will need to purchase a SIM card for the device.  Additionally, if you are using any data services, you will need to purchase GPRS or 3G data services with the card of your choice. Note: These GSM reference are only for users in the United States.

1. ATT GO Sim Card. (Amazon)

2.  ATT General SIM Services

        

3.  T-Mobile

        

4. Cingular 3G services

       

5. U.S. Tronic - Prepaid Sim Cards