vumatel/vodacom *****s peoples money

I downgraded my linespeed from 500 megs to 200 megs. They were supposed to debit me for R 1 500 together with my voip phone. They debited me with R 3 700 and can't tell me why. They say they are investigating but thats hogwash. Can someone sort it out for me. I get debited on the 3rd of the month and this is the only vodacom/vumatel debit. I dont have any other vodacom accounts.