What Is a SIM Card?

A SIM card is your subscriber identity module and it is the thing that can identify you. The idea there is that then they could be able to have some sort of query and I would have to respond. They've got a shared secret in their network that knows about you, knows your billing information, knows your phone number. A phone will get on the network and say, hi, I'm this subscriber and they'll send you a message that you then pass to your SIM card. It's actually a two way sort of handshake.