I have an odd problem here: my iPhone 3G is not allowing me to make any calls. It shows that I have service and the 3G data connection works (as well as texting I would assume). But, when I go to make a call, it kicks it back, saying "Call Failed." I am on firmware 2.2.1 (5H11), and the carrier is AT&T 2.9. I am not unlocked, just jailbroken. I have not yet installed the AT&T carrier settings update that was recently released. Is this the problem?
EDIT: The modem firmware is 02.30.03 (in case that might help).
If that doesn't work, and you jailbroke with Pwnage, make sure that you do NOT check "Activate" in the advanced options. Sometimes this will be a problem if you have a valid carrier in the U.S.