THE 2-MINUTE RULE FOR OPPO レノ A

I bought a next-hand mobile phone and when I wished to reset it, it requested for your password which I don't know. I checked the IMEI to see Should the cell phone was stolen and no, it is not stolen, but it does have an outstanding financial debt with the organization AT&T. So, I do not know what to do since I don't even hold the cellular's Bill.W