Dishonest Billing

I took out 2 contracts for myself and my son. To make a long story short , I closed my sons contract in 2014/5 and cancelled it as required. I renewed mine and the contract ended at the start of this year but I paid up and closed the contract in December 2016. I then receive a letter of demand from VVM collections saying I owe R1263.00 to Vodacom. I then request to know the origin of the amount for which they gave me 3 different explanations on 3 different occasions. I sent them my final payments which show that I owe nothing to Vodacom as mine and my sons billing was always on 1 statement. I then call Vodacom who then mails me a statement showing my account as being 30 odd cents in a credit and there was no further billing as I had cancelled the contract. I then went back to VVM who contacts Vodacom , who then sends them a breakdown showing a R1000.00 that was billed on my sons account which I ended in 2014/5. I was never sent an account , the amount never appeared on any statement and Vodacom never called me or made me aware at any time that such an amount existed. I then call their customer service , who refers me to legal , who refers me to VVM saying they cannot assist because they have no info regarding this matter. I am appalled at the total lack of service and dishonesty from Vodacom and feel harassed by VVM who constantly send me emails demanding payment. I cannot pay twice and I cannot pay for something that no one can give me an explanation for.