Blackberry Storm 9550 prepaid t-mobile question?

Okay, here's the deal.... my Husband and I bought an "Unlocked" Blackberry Storm 2 9550, we are trying to use it with a T-mobile prepaid SIM card... I checked and the phone is definitely unlocked, I switched it over to GSM... it is saying enter network MEP code to unlock SIM card. The guy we bought it from even sent me the code he used to unlock it and that didn't work. I called T-mobile and they had no idea what I was talking about and suggested I call Verizon. What should we do? DON'T SUGGEST YOUR UNLOCKING WEBSITE FOR MONEY.