My account is fully paid up

I called MBD around April to find out how much I still owe on Virgin Mobile acc and the agent advised me I was owing +-R1361. I had agreed to pay at least half of the amount and I immediately paid R701 and made an arrangement to pay the balance in May. Around May I received an sms from MBD that I must pay my balance which was +- R660 and on the 29th May I paid the amount to close off my account. A week later MBD calls to inform me I owed R181.22 for some charges that were not added to the account and now today another agent calls me to say I owe over R190. I refuse to pay this money because I had paid my account in full on the 29th May as per sms from their office. If they are trying to run a \Get rich quick scheme"at MBD then they must tell me so that I can sue them to Kingdom come. You wont hold me ransom for your own mistakes. I paid my account in full and you will close off my account immediately with no further calls from your office demanding money from me."""