Incorrect billing

I recently signed a contract with Mweb for R689 a month which was activated on the 20th of April and I was advised that the charge will be on pro-rata from then to the 31st of April. I received an invoice of R367.47 which I paid via EFT and then they debited R1056,47 at the end of the month. I sent proof of payment for the R367.47 and still have not received any explanation why I had to pay that much. Call centre is worse when it comes to billing queries.