MTN not trustworthy.

I cancelled my contract with MTN in June 2017. I did not receive any invoices or replies after that, so I assume it was cancelled. Now end of last year they all of a sudden call me and tell me that I owe them R539.00 I explained to them that the contract was cancelled in 2017, but they still handed me over to VVM debt collectors. The debt collectors let me know that MTN did not receive my cancellation, which is a lie because why did they not send me any bills since I sent them the cancellation letter.