Payment arrangement refused

Absa’s incompetency is really costing me my peace. Twice they did not debit when my debit date fell on a weekend after having made prior arrangements for them to debit the Friday. Now that the account is in arrears DUE TO THEIR OWN DOING. They refusing a months payment arrangement for me to cover the shortfall which is as a result of their system limitation, they are requesting that I pay over 3 months which makes a steep instalment for me. My kind ask to them was for the outstanding amount to be broken down into six instalments to make their mistake bearable to my finances. Now I’m being harassed by telephone calls on a daily requesting payment.