You have the option of having all the calls made to your iPhone routed to a different phone number, which could be useful if you are overseas or are in an area with no coverage for an extended period. To do this:
- Make sure you are in an AT&T coverage area
- Go to Settings >Phone >Call Forwarding
- Turn Call Forwarding on
- Enter the new number.
Your iPhone will no longer ring until you turn Call Forwarding off.