Harrazement for payment for oustanding cell c acc

Good day<br> I had a cell C contract which i settled in full and handed a letter in to cancel my contract but they keep on harassing me for money, They have handed me over to Munnik Basson Dagama Incorporated Attorneys I have sent them all my documentation, which includes the letter i handed in as well as the payment s i made to cell c reflected on my bank statement, but they just keep on sending me demand letters, i have tried to phone them but am put on hold for ages! I know i have paid them up to date but they said they did not receive my cancellation letter can you please advice me what to do as this is incredible frustrating and will never ever deal with cell c again. As the Sim cards belong to my kids and phones and cards were long gone and ******! My last payment to them was split in three and done 2nd August 2010 as i was unemployed and they agreed to let me settle my account in three and i have not herd another word from them since 24/03/14 as they handed me over to the lawyers mentioned!!!