Cancelled contract In March but still being billed

I cancelled my month to month cell phone contract with Vodacom on the 3rd March which would mean that my final payment for my march calls would be in April. I received an invoice for May payment and I have been going back and forth with the customer service division on the matter. No one has checked the so called voice recording from the 3rd March & it is very frustrating! I have not used the phone number since 12 March at 1pm as I flew overseas for work. I called the cancellations dpt. number & requested the cancellation of the contract. The customer service employee asked me a few questions and confirmed the contract will be cancelled from 31 March! Now the cancellations department are asking me for the written notification. I have stated on many occasions that I called the cancellations department telephone number not emailed! Why have a telephone number for cancellations if you cannot use it!? Why has no one bothered to go back to the voice recordings for the 3rd March? At the start of the call it states that all calls are recorded. This is very frustrating & actually a ******* of money! I am being penalized for an employees inefficiency to do their job.Pls contact me via email