Impossible to cancel Telkom accounts

Our company has canceled their ADSL with Telkom of which they confirmed receipt of said cancellation which was sent to them in writing, they have now continued to bill our company three months afterwards. I have everything on record. This has happened to me before with my personal account as well, which they finally canceled almost a year after they handed my account over for collections and listed me on the credit bureaus. Telkom attempts to collect on accounts that were canceled and I wonder if they solemnly cede these accounts over to third parties, which would be *****.