Telkom
Debit order processed after changing to pre-paid

I converted my account to pre-paid a few weeks ago and thought that the debit order would automatically be cancelled (as it should have been). Telkom deducted R500 but my account is in credit after I topped up with vouchers. Why they deducted so much I cannot understand. On calling them they say that it will take a month or two for a reversal. This is unacceptable as they should never has processed the debit order. This money can be paid back into the account but telkom is being unethical.