Support can't find my 360 SN

Hello , I have a big problem. I can't remove parental controls from a used xbox 360 because xbox support can't find my console. They say my serial number is non existant. So 1st time iv'e called a guy told me by 24h a will received the code by email and a upper technical support will register my console, but now a week later iv'e got no reponse. Then iv'e called back and the girl said to me she can't do nothing , and finnally she says even is manager said to me they where unable to do nothing and then Iv'e said to her what happen last week and she told me that my case as to be post poned at a USA place because they have more technicals tools, and say she will call me back But Iv'e got no call back. Iv'e tried to register my console by myself on the microsoft account and it say to me they can't find my serial neither. And the person i buy it from was not the the one who set up the code and she buyed it from a retail store and so I have no way on earth to find the person who set up the parental console lock and I don't know what to do. Iv'e never experienced that and the xbox support neither. What can i do ?

:)