Being charged a device cancellation fee for a paid off device

I would not recommend MTN to anyone. I fully paid off a device for two years, never missing a single payment and submitted a 30 days' notice to have the plan cancelled when it expires. They are now charging me a device cancellation fee and claim I cancelled before the expiry date of the contract. I have emails from their representative showing that the contract was cancelled on the expiration date. This is truly appalling service from MTN, and they've lost a customer.