When you buy a new iPhone, you probably want to transfer the phonebook listings from your old phone to the new one. Fortunately, phones that use a GSM-based network like AT&T, T-Mobile, and most of the rest of the world, use SIM cards to store phone numbers and contact info. To import contact info stored on another SIM card, follow these instructions:
Remove the iPhone’s card and slip the other SIM card into the iPhone (instructions in user guide).
From the Home screen, select Settings >Mail, Contacts, Calendar.
Tap on the button labeled “Import SIM Contacts.”
If you have Contacts enabled for both MobileMe and Microsoft Exchange, you are prompted to choose which account you want to add the SIM contacts.