Over billed not very thrilled

I have an uncapped WiFi (R319) 2 Phones (R199) and 2 routers (R149 each) per month . Recently mtn called me and asked me if I wanted to upgrade my uncapped WiFi package which I declined . I was happy with the service until all of a sudden a debit order for around R2500 was deducted for the month . I checked with *141# what my due balance was and it was more than R1100 . I did not tell mtn to change my package and I was not informed that it was changed which is ******** to do on my behalf . I am going to retract the debit order and cancel this service .