INVOICED FOR CONTRACT PHONE THAT WHOULD HAVE BEEN CANCELLED

I HAVE BEEN STRUGGLING TO CANCELL A NEW CONTRACT WITH MTN AND MONDO SINCE JANUARY. THE PHONE HAS BEEN SEND BACK ON THE 30TH JANUARY AND STILL I GET INVOICED FOR IT. I DONT KNOW WHAT ELSE TO DO TO GET THIS SORTED. I HAVE ALLOWED A DEBIT ORDER ON MY BANK FOR A CERTAIN AMOUNT, AND THE INVOICE GOES OVER THE ALLOWED AMOUT, MEANING THAT MY OTHER TWO CONTRACTS WILL NOT GET PAID. CAN SOMEONE PLEASE SORT THIS OUT!!