Paid up my vodacom contract but am handed over to attorneys.

I had a vodacom contract were you get two phones. I have requested a settlement letter (August 2017) from vodacom because I wanted to pay my contract and close it. they did sent the invoice with the settlement amount and they said invoice is valid for 5 days. I paid within 5 days and emailed back the invoice with proof of payment. Surprisingly when I went to vodacom to do a swim swap they said they cant help because am owing vodacom, I had to call the head office after a long conversation with the call centre team leader, he apologized and promise to correct the mistake and I was allowed to do a swim swap. after some time I received a call from MBD saying that my vodacom account was handed over to them and I must pay R970. I am so disappointing with how vodacom treat customers, because now am handed over to lawyers for the account that I have paid on 25 August 2017. I do not think that I will ever do a contract with them or advise a person to deal with them.