I've had Cingular service for the last 5 years. I ordered a new phone last year from the Cingular website (renewed contract on the same service plan) and the phone came with a new SIM card. But I just took my old SIM card from my old phone and plugged it into my new cell phone and it worked just fine, with all my phone numbers still in memory. What do I do with this new SIM card, which I never used? Did it come with all my information/phone number preinstalled? If I gave this SIM card to someone else, how do they go about getting a new service with it?

I apologize that my knowledge of SIM card and related technology is rather limited.