Mtn is misleading us

I app**** for 2 cellphone OPPO contracts and they told me that I am going to pay R699 but to my sunrise I was debited R2600 I went to the store in Maponya mall and I spoke to the manager he told me that he was fixing it and he will refund me or he will call me on the 15th of July he never called me back or I never heard any word from him... Now this month they debited the very same amount of R2600 again so I have been going up and down to Maponya mall Mtn store and I have spoken to the manager but with no help they are not treating customers fairly what we sign for on the contracts is not what we get