During covid I continue to pay a lesser amount on my Sterns account

During covid I continue to pay a lesser amount on my Sterns account. I made an arrangement to pay a certain amount per month and the payments made on the 6th of each month. Every month I get a call or sms asking for payment by the end of month. They also charge you R20 for each call they make to you if you answer that call. Their call centre staff are absolutely useless taking messages down verbatim.