Overbilled for R799 Deal

After taking the R799 deal i was told i will be charged Pro-rata for the first month. As i took up this deal on the 6 March the Pro-rata amount works out to R644. i was billed R850 and no one can tell me why. Call center is useless and keeps sending me to the department that handles the DPD. One consultant said its correct the pro rata amount? When I took the offer i asked and sent emails on several occasions to make sure the billing is correct. I was assured it was. Can someone competent at multichoice sort this out?