I googled "affirmative defense" and in my particular case, I think you mean, if I get sued for a debt that is past the SOL I raise the issue of SOL and that is all the defense I need, it is a solid legal reason for not paying the debt and the plaintiff would lose.
However I am not looking for a defense in court because I do not expect to be sued for a debt that is beyond SOL. The question I raised was whether a debt collector can still call after a debt is beyond the SOL and I haven't read anything here (or linked from here) that says they can't.
I did check out the link provided and it seems to deal more with starting the validation process when a debt collector calls.
I have two things going on:
1) debt that is past SOL - I occasionally get calls but it's not collectible and I am not worried about it (it's going to roll off my credit report in about 15 months, I hope - if it doesn't, I'll deal with it then)
2) debt where the creditor has gotten an arbitration award - I will probably try to negotiate a lump sum settlement, and then claim insolvency with my tax return so I don't have to pay tax on the "forgiven" amount. I have researched this exhaustively and know exactly what is required in this case, and am prepared to do it (as long as I can negotiate a reasonable settlement).
Last week, a debt collector on the amount with an arbitration award sent me a settlement offer - today I sent back a certified letter with return receipt requested saying I disputed all or part of the debt, and requesting verification of the debt. This is not just a maneuver, I actually don't know how a debt that was $34K two years ago can be up to $61K today.