Call them and see if you can pay them $20 a month. If that fails, get hold of a consumer action attorney, and have them contact the ambulance service, and ask if you can pay them $20 a month.
Businesses, such as ambulance services and hospitals, would rather recieve $20 a month, than have to hire a collection agency to get the money from you, which more times than not, doesn't happen.
Did you charge this on your credit card ? If so, pay off what you can each month. One thing you may want to consider is contacting your credit card company and asking them to lower your interest rate. Believe it or not, often they will !