Overcharging my cell phone contract account

I am Mashudu Davhana have complain with Telkom Mobile. On November 2015 I receive a phone call from Telkom Mobile about my contract which is due for upgrade and agree to upgrade my contract to R179 per month. on the 10th November they send my new Samsung galaxy core prime, to my surprise on the 15th December 2015 they did not debit their money, on the 20th December 2015 I phone Telkom consultant to find out why to my surprise they tell me that they are going to debit R852.28 instead what we agree upon. They promise to resolve the issue within 72hours but unfortunately to date nothing has being done. on the 30th December 2015 they send me massage that they are going to debit R852.28, then I phone them again and ask them not to debit that amount as on my account I have less than that and the bank will charge me extra charges. They promise to reverse the money but instead today I receive the message that on the 15th January 2016 they are going to debit again more money which is R528. please assist me to get my money back and also to be charge as we agree or to cancel the contract if is difficult.