my ousin is main account holder . he let me use an extra ph. line w at&t

so i upgraded the phone line at best buy and filled out the upgrade paper work and gave them my cousins info so i could be   the secondary person for the extra ph. line . so now best buy tells me im not set as an secondary person . so i saw the person that upgraded me  and im asking for a copy of the upgrade papers  so i can get my cousins info back and do it my self bcause now i worry about that info and best buy personel says they cant help me