Received a bill for over R2000 on a prepaid contra

Have been trying to sort this out with MTN for 3 months. We have a TopUp100 for R79.00 per month. So once free minutes depleted, we top up, so that we can control budget. Phone purchased seperately at Edgars, without contract. Bill has averaged R79.00 per month, since beginning, until April when it was R974.40, then May R2402.92. When queried, MTN was striking & could not send me copy of itemised bill or even copy of contract change/upgrade that may have occurred. Several phone calls to MTN and we still cannot find out how this happened. The phone was suspended this week, with still no explanation from MTN as to why the TopUp account was suddenly open ended. And it is now back to R79.00 average a month. We are now being threatened with legal. This is unacceptable and preposterous. Today we were told to go back to Edgars, because, perhaps they have a copy of a contract that we signed! Why doesnt MTN have this supposed contract? Because it doesnt exist.