[tr0s]

first off i want to make sure we are talking about the same terms here. your phone needs to be sim unlocked. (the unlocker you have to pay for). when you do buy it, it's a 3 stage process but it will not only unlock your phone to be able to flash roms but it will unlock your sim so that you can use an ACTIVE at&t sim card and it will recognize the network. i'm pretty sure you don't need to "activate" a phone on at&t's network, just make sure the sim card is active and you should be good. once you're sure that your phone is "sim unlocked", and you have a good sim card in it, go into settings and switch the phones band to gsm and you should be good to make and receive calls.