Invalid claim on account

I have been frustrated for months now with MTN claiming I owe them money for a contract I cancelled in 2014. I have provided proof of payment as well as a reference number and name of the call centre agent from their Corporate Division that I spoke to. My number was cancelled and converted to prepaid. Now out of the blue I have been receiving communication from a collections agency, Hammond Pole, who have been instructed by MTN to collect the money on their behalf. Every time its a different agent that will communicate with me and none of them ever pick up from where the previous agent left off. I have provided details of my contract cancellation to them and they refuse to go back to their client, MTN, to request the recording of the call I made to MTN to cancel my contract. I am at my wits end now with this debacle. This matter has been handled very poorly by both MTN and their agents, Hammond Pole.