Unprofessionalism and Incorrectly billed on the Month to Month deal

I have never signed any contracts with Rain, the only package I took out was for month to month sim card data. To my surprise now they saying I signed a contract with them and there is an outstanding fee of R500.00. The outstanding balance how was it accumulated on the month to month package as if the premiums was not paid there was no connections. This is unacceptable as feels like the billing department is not doing their jobs, as you cannot billing someone with the month to month. I want this to be resolved ASAP as this been going for too far and if i am not assisted this would be escalated further.

please provide feedback urgent and my email address is *** and contact number ***.