Oh its an att iphone?
You fooled me cause you said clean ESN so that automatically usually means CDMA phone.
GSM phones like the AT&T iphone don't use ESN's. They use IMEI's.
Ok then bring it to them and see what they can do for you.
I don't know why they would need it GSM unlocked since their network is not GSM anyway but they should be able to get you more details.