Yes, you'd be surprised how often they have no paper work. If they can't prove you owe the debt they can't collect.
Here the letter I've used in the past:
Sample Letter 9 - Requesting a Collection Agency to Validate a Debt
Make sure your letter states they are not allowed to transfer you debt to another CA or you will sue them as well.
Seems pretty solid and then you can just negotiate the rest and try to get it removed for settlement / full payment.
Also should I setup a P.O. box for mailing these?