If I buy a cellphone that is unlocked will I be able to activate it with my current carrier??

Answer 1

if it is gsm based then you can swap out the sim card and use it right out of the box (assuming it is charged up).
i'm on cingular and i have bought unlocked phones that were branded with t-mobile. sometime you get features that are still included with the old phone that will not work with your carrier.

Answer 2

Yes,Just put your old Sim card in the phone.