In your other thread you never gave the exact model and firmware version, which makes it difficult to give good advice.
We deducted it must be a iphone 4 because of the 04.12.01 Baseband.
On the iphone 4 iOS 6.0 you can tether jailbreak it and activate during the progress. It will still be locked to AT&T though.
Tutorials on how to do it are on iClarified.com
Read up on the consequence of a tether jailbreak before you do that.
As suggested in your other thread you can also buy an IMEI unlock, that way the phone is unlocked and activates automatically.
You can also buy a used or inactive AT&T SIM to activate it, it will still be locked though.