iPhone 3G, upgrade to 4.2.1 = No Service

[planktonika]

Hey guys,

I upgraded my brother's iPhone 3G to 4.2.1. Everything works, except network access. It's an AT&T phone that now has a prepaid AT&T card. I made the custom IPSW with pwnage tool, hacktivation on. When I tried to upgrade on his PC for the first time I had a 1600 error, so I used iREB to make it work.

How can I fix this? What's probably causing the problem?

Thanks.

[iYeow]

Build another custom firmware with no hactivation, let your ATNT activate, you will now get service.

[R.Mortera]

If its an AT&T phone, why would you hacktivate it?