Who was the "letter" from? Is it a statement from the debt collector of intent to sue, or is it an actual notice from the court?
If from the court, it should specify the period, substance, and manner of any response reqired by them.
Meanwhile, you might start thinking about a defense, the obvious one being a motion to dismiss if the debt is outside of your state SOL.
In my opinion from my experience, they are just trying to force a settlement. I don't think you want a judgment on your credit so call the collector and see what you can work out. I just had the same situation for an old Capital One account that was sold to Kramer and Frank. They may tack on some additional court fees to your balance. I was still able to settle for 60%