Still paying account 2years after expiration

Approximately 2 years ago, I my contract had expired with Cell C on a tablet I had purchased. after MANY months of battling with the accounts department and their outsourced collectors (NDS) I had finally closed my account and OVER paid my outstanding bill with both CELL C and NDS, this cost hundreds of unnecessary rands. yesterday morning I received yet another sms from CELL C informing me that I have been handed over to NDS, its been now THREE FULL YEARS now that I have been out of this contract, this incompetence is exactly why I moved over to Vodacom. bottom line is I have proof of payment and letters of paid in full to prove that I have made right will my creditors, WHAT ELSE DO THEY WANT??<br> <br> Mark