Sending strange people to say I owe when I paid

When i cancelled my contract I was told to pay a certain amount I did, and everything came to 0, suddenly i'm getting calls from vvm, first 100 times noone says something the phone just hangs up, then i'm told i owe 700 something odd amount, I ask how, when I was sent an invoice and paid, this guy tells me to contact vodacom, so here's my thing, for invoices all this time vodacom knew where to reach me, suddenly I owe them but they never sent me an invoice to confirm this, i just owe them out of the blue? and the approach they take even? so now the phone cut and i dont know if i owe them or not but I see why other people dont think twice before jumping ship to other networks, client service at vodacom is sitting at 0