DOUBLE CHARGE and INCORRECT INTEREST

fnb failed to debit their minimum required installment from my account on the 25th, as per agreement, and when I paid this amount later during the day at about 17:50 I called them to confirm receipt of payment and that indeed it was paying towards their mimimum payment. This was confirmed, yet on the 29th I was charge AGAIN and with interest nog al. I called premier banking twice, and got disconnected with their online 6 digit and pin thing. MIND you I am spending my my airtime here and no-one called me back