Overcharging on Top up account

One of Sales Consultants phone me regarding a upgrade on my current contract, he told me that if i take the contract the only amount that would be deducted from my bank account will be R173. Not anything more as I explained that I didnt want to pay more than Im currently paying (In October 2014). In November I received an SMS saying I owe R1500 unbilling on a Top Up contract of R173.00. I took out my sim card and didnt use it since then. Now at the end of the Month January 2015 R3627.00 was deducted straight from my bank account, without my knowledge or permission. How can they say its only going to be a certain amount and then rip you off without even contacting you. I contacted them on the 1 st of February 2015 so they said that data was used on my number so I explaint that the sim isnt even near the phone. Now I must sent an email so that they can check, To how many people do you need to talk before you can get helped. Very quick to make promises but dont keep to the amounts agreed upon. Its not the first time and its really making me mad to know you've been with a company for so long and then they do something like this. Its unprofessional and really disappointing.