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Once you get to court and they have not validated, you can ask for it in court. I think when it happens in court it is called discovery.
I would take a copy of the DV you sent and the green card (did they sign for it) along for proof.
Also, how old is the debt? Do you know the DoFD?
That is a big difference in amount if it was only CO'd a couple of years ago.
I was hoping you could claim expired SOL but GA is one of those states where it has been ruled as a written contract (6) instead of an open account (4). Other judges may go by the 4 year SOL.
Whether or not fees and interest can be added depends on the original agreement and your state laws. Ask that the collection agency break down how they arrived at a $1200 debt.
I would also let the judge know that you tried contacting them numerous times and the number they provided was not a working number.
If someone is going to sue you, you should be able to get in touch with them if you needed to.