Owing mtn without my knowledge.

I had a month to month with mtn where I was paying R349 per month for data. Now I stopped using that Sim card last year since I no longer need the data, I got a router for the whole house. Now I am receiving calls from mtn saying I owe amount of plus minus R2000 for that Sim card that I am no longer using, like I said the data was month to month, it was no a contract for 12 or 24 months. So I dont understand why u should be owing them that much while I only took month to month option. At this point this money they say am owing is affecting my credit record negatively that I can't even apply for a car finance or for a bond. I am no longer using the Sim card or the data so there's no need to pay as that was not a fixed term contract it was a flexible month to month