From what I understand, you've had 2 previous payment plans, and when it became obvious you wouldn't be able to pay, you contacted the company BEFORE a payment was missed - is this right?
If so, I would take it higher in the company, on the basis that you are happy to enter into another payment agreement, and you have followed their terms for payment agreement in the past. It is simply impossible for you to get $600 in the next couple of days, but if you can do a payment agreement, they should be taking it.
If that fails, I would take it to the Ombudsman - the issue is not whether or not you owe the money, the issue is that they are not allowing you to pay the amount you are able to, and are demanding the full amount in an unreasonable amount of time.
PM me if you need to - I've had experience with this type of thing :)

