RSAWEB Billing error

RSAWEB made a mistake on their side with my billing by invoicing less than I had to pay. Fair enough its money that i would have had to pay anyways but now they are expecting me to pay the full amount outstanding in one debit order which amounts to more than double my normal payment. after speaking to someone in the billing department to make a payment arrangement i was told they can only do it over three months, still fair but I was never shown a breakdown as to where they got to the amount now i must just pay it based on a figure they showed me. Just for the lady in the billing department to put the phone down on me while im still giving my part of the whole story. This is totally unacceptable and poor customer service.