Telekom contracts ended but I’m still billed

I had 2 cellphone contracts with Telekom, they were paid off before the contract period was over. I advised the consultant that I would then continue on Telkom as a pre paid customer. Thereafter I started receiving a bill that kept increasing monthly and was advised that it was due to a debit not going through and they put the charge on me, to date one from Telekom has contacted me vis my escalation. I am now being harassed by a legal company HP Attorneys of whom I have explained the issue to and they keep saying that they will look into the matter and nothing happens. Theses calls and sms's are really getting out of hand and I need answers from Telekom at Management level because their consultants are extremely rude.