BLACKLISTED YET I HAVE SETTLED MY ACCOUNT IN JUNE

HELLO PETER. I HAVE A PROBLEM WITH CELL C , I HAD A CONTRACT WITH CELL AND DUE TO THEM NOT DELIVERING ON THEIR PROMISE I OPTED TO TERMINATE THE CONTRACT. THEY TOLD ME OF THE COSTS AND HANDED ME OVER TO NUDEBT MANAGEMENT COLLECTIONS AGENCY. IN JUNE 2014 I SETTLED THE ACCOUNT WITH THE AGENCY AND THEY PROMISED TO SEND THE PAID UP LETTER TO CELL C WHO WILL IN TURN UPDATE MY STATUS WITH ITC AND EXPERIAN. NOW WHENEVER I TRY TO BUY SOMETHING ON CREDIT THIS DEBT COMES UP AS UNPAID. PLEASE HELP ME