Unacceptable levels of service from MTN

I opened a data account with MTN in January 2014, I cancelled this data account in October 2016. MTN continued to debit my bank account until I reversed payments and put a stop on the debit order. I've now been handed over to MBD Incorporated (attorneys) who are claiming that I owe MTN money. I've spent 2 weeks trying to contact MBD and MTN to try to ascertain where the apparent debt comes from but MTN can't help because I've been "handed over to the legal department" and MDB can't assist because they don't have any information on the cancellation of the account. PLEASE CAN SOMEONE ASSIST ME WITH THIS?!