Endless phone calls about debt that I do not know about.

Its been three months now that I have been getting calls from Vodacom from the ********** 708. They are alleging that I owe them R300.00. I only use one phone from Vodacom and the number is ********** 619 which is a prepaid. My last contract with vodacom ended in June 2015 and I made sure that it was closed. I am using this number to make phone calls and access internet via my computer and I have been paying for this cash. I am sure the network has the records of how I pay for my airtime and data for this number. I do not have another contract, and do not understand why i get calls from the network threatening me with legal action.