Poor comunication

My contract is coming to an end and it was said by two agents that I can fill in the cancellation form and send it through to Telkom. Since I have done that I have not received any feedback to say they acknowledge my email or they are in the process of cancelling the contract.

This is really pathetic because I need to give a months notice, which I have done, but have not received any feedback. So they can easily turn around and deduct next month and say they did not receive any cancellation letter. How do I as the client protect myself, and show proof that the company received the cancellation letter?