mtn (mbd)bad service

i had a mtn account for mobile data,which was handed over to mbd(munnik basson dagama).i have been making payments and sending them proof of payments to the finance department to a lady which deals with updating payments,her name is Flora Mohola.According to my payments,my outstading balance is R836.00 but they say its more than R1500.i have called and sent emails numerous times but no one bothers to assist.