Unauthorized debit order of R4900

There's an amount of about R4900 that debited on my account in 2020 whilst i was still paying my normal contract of R548 with Mtn. I called their call center and they said that they will escalate the account in question to their ***** department to investigate it because it was not theirs. For some time after that i relaxed thinking that it was resolved because it was no longer debiting, and their ***** department never got back to me with an update of what really happened. Now Mtn has handed me over to mbd associates for an amount i never knew about, because i continued paying the R548 contract until i was done with it and asked them to cancel the contract afterwards. Today I'm busy making calls to Mtn so they can clear my name on bureau, I'm not getting any assistance I've been asked to hold on while they check my account history and it's been 2 hours now waiting for the person to get back to me. I don't know who is going to assist me with this if not them because they need to make me understand what is this R4900 that they want me to pay for!