iPhone 3 Upgrading and other similar issues
No one has responded, and with the information you've given, it is difficult to know exactly. But I would guess that while jailbroken, the device was also unlocked, and when you restored factory settings, you lost the unlock, and now the iPhone is once again locked and expecting an AT&amp;T SIM. If you can acquire an AT&amp;T SIM, and it works, then this will be confirmed.

However, this is a rather common issue with a very simple solution, and perhaps my hypothesis is wrong. 

Sorry it took so long for a suggested solution! 

And Happy New Year!