[err02a]

I recently purchased an AT&T 8310 from a friend. phone works fine, or at least did for him. I am using the pay as you go version of AT&T, i've used my SIM in the past in PDA style phones, but when i drop my SIM in this phone it will only get as far as SOS signal. I've hard reloaded, updated to newest OS, and even purchased an unlock code to try and fix. any one have any suggestions??? thanks in advance