Harassed, even after settling debt. No proof.

In November 2014 I was contacted by a person from MBD that informed me that I owed Cell C R329.23 for a contract I had with them from Nov 2004 to Nov 2006, and that I must pay this immediately. I asked for an invoice or statement to prove this, but never received anything. I was not sure if this was a hoax or not, so I contacted Cell C who confirmed the debt and I subsequently paid the amount owed to Cell C (even though I believe this debt has expired). Cell C could also not explain how there was an amount outstanding since I had a debit order in place for this contract. About two weeks ago I received another call from MBD to confirm that they have received my proof of payment and will update their file accordingly. Since then I have received two emails threatening legal action if I don't pay, and another call from MBD to inform me that I now owe R411.00. When I asked for an explanation of what the R411 was for, the person from MBD could not explain. I again asked for an invoice or statement to see where this amount came from, but to date have not received anything (except another threatening email). Nothing is clear and transparent, and no supporting documents supp**** yet.