Cancelled account - Consumer Complaint for ADT Head Office

I contacted adt and said i need to cancel my account as i am moving and the new owner will take over contract i done all paperwork as i should and mailed it back about 5 to 6 years ago maybe longer and now the last few months i keep getting calls from colections companies stating i owe almost R8000 so how is it my problem if they dont do their work as i will not pay it and i will not resend the paperworks as i dont have them any more so if they want their money they must contact the owner of that house now to get their money