I just purchased a tmobile 5s and decided to stay with ATT. now the question is, are all of the 5s tmobile unlocked? what could possible go wrong with me purchasing this and the seller may screw me up? let's say the phone is still under contract and the user didn't pay his obligation but he sold the phone to me. can the phone be blacklisted and screws me over? can i unlock the phone myself even tho i'm not with tmobile? can someone explains imei, esn etc to me, i've been with att and i thought those are for cdma phones like verizon and sprint. is that true?