Overcharged on contract

Am very much disappointed with how Telkom is running their business. I took a contract for a Telkom router which comes with 10 gig a month for R139. Every month am over charged and had to call their call center to rectify their mistakes. On Friday the 15th they came in with a shocker, they deducted R619.00 Andi called them again and they referred me to their offices in Thavhani Mall where I didn't get help on that day as they told me that they need my Identity document with so that they can be able to assist me. I went there again today with my Identity document as I was told last week on Friday and the lady who was helping me told me that she doesn't know why they are deducting so much money from my account but she can't help me in getting my money back. She just told me that if the problem persists I've to come back to them again which to me is useless as they failed to help me. Am gatvol with Telkom. It's like they are in the business of robbing their clients.