Exceeding billing cap

I entered into a cellphone contract with Nashua Mobile in 2013. I capped the total bill at R400 p/m. Although the account cannot be accurately capped at exactly R400, the account dramatically exceeded the capped amount on several occasions. After all the Nashua accounts were automatically transferred to Mtn directly, I received my latest account from Mtn at R787.06. After enquiry with Mtn I was advised that the cap of R400 was never transferred to Mtn, which is of course an error on their side. Mtn customer service