Wrong billing

I have 2 contracts with mtn which I'm paying R420 a month, in January 2021 someone called me to say they want to upgrade my contract, I will get 20gb day and 20gb night at R400 for both, I'm going to save R20 from my previous contract. Only to my surprise I was debited R1007,I'm not sure what is it for. I don't understand why I was debited this amount, I need this amount to be rectified asap. Thema.