No service whenever I leave town

Whenever I leave the Denver metro area I loose service completely. My son has an iPhone with ATT and his will still have service wherever we go. I've tried turning off 3g, rebooting, resetting network settings, resetting all settings and finally I did a full restore via iTunes. I've also tried switching SIM cards with my son. My SIM card works in his iPhone but his does not work in mine. I've also called ATT to confirm my account is current as well as any settings on their end... Nothing and more nothing. Any ideas? I don't understand why it would work fine everywhere near home but not at all once I get a few miles outside the city limits. And still not at all when I get to a town that has an ATT tower.

Any suggestions are greatly appreciated!