Yes you can unlock your blackberry 8800 by unlock codes and for codes try from your provider or try from online providers..try
mobileunlocksolutions to get unlock code for your mobile and get it unlocked from AT&T network and use your T-mobile SIM in your unlocked mobile..before unlocking your mobile you can get clear instructions about how to unlock your mobile for that you can visit
mobileunlockguide to get step by step instructions..