I have a third party unlocked GSM phone.<BR>I want to use it with a pre-paid plan.<BR>I'm considering Virgin Mobile as my option,<BR>since their rates are good and I can easily<BR>buy top-off cards.

A SIM card, or subscriber identity module, is a small card in your cellphone that connects you to the network. Your SIM card contains your phone number, and lets you make phone calls, send text ...