Standard Bank and Stupel & Berman

Hi I keep on getting Messages from Stupel & Berman this month saying I must pay my Standard Bank account. I called them an enquired why Standard Bank handed me over to lawyers and they said I was in arrears but I made an arrangement when I was unemployed to paybthem monthly whatever I could.

According to Stupel & Berman if I don't pay them R4000 today and increase my monthly payment to R500 they will hand me over to the courts and will send me a summons.

It is so stressful when u are trying to show good faith and still trying to pay them what u van each month but they keep increasing the demands.

Is this legal?