cancelled contract

We had a vodacom contract for many years and in March 2014,we cancelled it.what I do not understand is that more than a year later their collection department start pestering me with calls and sms that we owe them R800.After numerous calls to Vodacom and the collection department no one seems to know what's going on.I will not pay them any money and if they do not clear my name I will take it a step further