Just a warning on cancelling the debit-order...
I had the same problem, where I cancelled the line 4 times (3 faxes and email). Telkom did NOTHING about them. So I cancelled the debit-order.
6 months later I get a letter from a lawyer saying they are going to take out a judgement against me on my credit-rating because the line was cancelled but only after 4 months, and I have not paid 3 months of the account.
I explained the story and they said if you read the cancellation fine print, it says you are liable FOR ALL COSTS leading up to the cancellation of the line no matter how long the line takes to be cancelled.
So if you cancel at Month 1, and Telkom only finally does the cancellation at Month 8 - you owe them 7 months of payment whether you used it or not.
So I had to pay the amount. It was frustrating but unfortunately thats all you can do. So yes, even if you've cancelled and they are debiting you - they apparently have the right to do that up until they deem the line to actually be cancelled.
Don't think that would not hold up in court as you went to all reasonable lengths to get your line cancelled, just go to the small claims court.
EDIT:
Also, the date of cancellation is when you handed in the notice + 20 days/end of next calendar month, Telkom specifically states 20 day notice period.
"4. A customer may terminate an agreement/service at any other time (i.e. prior to expiry of the term) by giving Telkom twenty (20) business days written notice"
http://www.telkom.co.za/today/media/downloads/42546_Contract__Service_Cancellation_form_F_2.PDF
And by sending it to their official mail, they receive the notice. Not your fault if they don't check the mail.
Otherwise I'd sign you up to any contract I feel like and just ignore the cancellation notice, thereby I'll just keep making money.
Do note that lawyer is from Telkom btw, so he will argue to get you to pay, the moment you pay you acknowledge the lawyer is correct.