Cancellation Mailbox Full

I have tried to send in my cancellation form to ensure the 2 year LTE contract does not continue past the original expiry date and become a mont to month issue. However the cancellation mailbox responds with an undelivered message due the mailbox being full, so how am I suppose to cancel the contract? This has to be violating a law on CPA as there is no open channel to submit a cancellation request (due to the full mailbox at Telkom) for a contract that is due to expire in 30 days and will then go on to a month to month contract without my consent.

********** The recipient's mailbox is full and can't accept new messages at this stage. Message therefore not delivered. Please try resending this message later. [Telkom SA SOC Limited. Registration nr : ***]