Overcharging on upgrade new contract

I upgraded my cellphone contact with MTN in April this year. Since then, I have been overcharged on my account. The administration on MTN's behalf has been completely ***********. I have spent over 6 hours on the telephone attempting to sort it out. To which they can only log a query and you can only hope that it gets sorted out. I have been into the store twice now to sort out my account. I chose a contract of R349 per month without a devise and have been charged R545. This was the same with my data contract that I signed for of R119 and have been charged R194. MTN are very quick to debit accounts and make the process to query accounts extremely unpleasant. I have found this to be extremely frustrating. After being loyal to MTN for 15 years. I will be considering migrating the next time round.