bad service from Cell C legal department

This is to cell c legal department.cell c handed over my account to MDB inc. for collection and the debt collectors contacted me and i had an arrangement with them to pay the account,after i payed the last instalment they informed me that there was a second account which i knew nothing about.i told the debt collector about this and they said i must contact the cell c legal department. i have been trying to get a hold of them for the past two weeks their phone will ring for 20 minutes and no one picks up the sad part is i am applying for my bond and this is holding me up as it appears on my credit rating and profile i will not pay an amount of R4000 i know nothing about.