Line still not cancelled

My dad cancelled his landline in April 2018 at a store, they gave him a form to complete, witch he did, they told him it will be cancelled at the end of April 2018. We now in October and the line is still being billed. I have proof of the form that was signed at the store in April but cannot send it to telkom as both their email address on the cancellation form does not work. Both return. How must you deal with them if their email addresses that is on official documentation does not work, the account must be credited from May 2018 to date and service cancelled?