Unresolved issue - threatening legal action

I sent a request to Telkom to cancel my service in November 2016, as back up I did not pay my subscription for that month encase they never cancel the line and I would be obligated to continue paying. I have spoken to various agents at Telkom and I keep emailing the cancelation form but never receive confirmation of cancellation. Telkom have now taken me to debt collectors and are threatening to blacklist me but my issue remains unresolved. I have a mailing trail prove that agents do not resolve the issue despite me following up to ensure I pay the correct amount owing to Telkom.