Debt collection: car accident with no insurance?

So, I was in a car accident recently, and, at the time, I did not have auto insurance. I went to my mandatory court date and presented proof that I now have auto insurance, and I was let off the hook with only court fines. However, a collection agency is now calling me, demanding that I pay $6,000 for damages done to the other vehicle (which was insured by Allstate). What do I do?