Billing cycle

My contract with MTN is +/- 15years old. Last month i made a payment on the 25th as i always have with an overpayment of R1. Suddenly this month on the 23rd of April i get a notification that my account is suspended. when i called mtn the lady insists that my account is suddenly on a 21 day cycle after +- 15 years, and that i need to make an arrangement. Apparantly it is like this because i pay cash, i dont see how mtn can just change my billing cycle because i pay cash, and since when does a contract demand payment of funds before a client receives his/her remuneration. All the lady was interested in is me making a payment arrngement, which i cant understand why because i have always paid on the 25th. I had asked to speak to a supervisor and after 21 minutes i was still not attended to as the supervisor was on a call... is there only one supervisor / team leader on the floor for such an huge company? and since when does MTN just change their billing cycle where do they expect us to get funds fromf. they may change their billing cycle but our employers dont change our remuneration date....Disgusting