Still being billed after canceling account

On the 8th of June I cancelled my Telkom account by filling in the cancellation form and sending it to the instructed email address. As per their terms I sent this more than 21 days before the cancellation date. On the 12th of August I once again received an invoice from them. I phoned the accounts department who asked that I forward the cancellation email and that they will look into it. On the 13th of September I once again received and invoice for an even bigger amount.

I cancelled in time, followed all of the instructions and still am I being billed for a service I do not use or want.