Dispute over an unknown contract

I'm due for an upgrade and Telkom tells me that I have a bill that I have written off the bill for R1,068.47 (***) for another contract which I do not know of. I made a police affidavit to support my story but they tell me it takes a long time to investigate such cases meanwhile I'm paying the contract I'm aware of which amounts to R917.00 p/m (***) and I've been their loyal client for many years.