Increase in monthly installments

My wife and I signed up with Vox on the 18th December 2020. After back and forth with their sales department trying to get the line activated, the line was finally activated on the 20th February 2021 by the assistance of Kevin... thank you Kevin. However today we received an email saying that our monthly installed will be increased by +- R170 a month. I have not even paid my first installment yet I am getting an increase??? I called the accounts department and they advised they increase all customers with vox in April even if you have recently signed up with them as this is their annual increase period! I am not even active with them for a month and they are increasing my monthly installments.