The notice you sent 5 days from receiving the original notice in December 2005 constitutes your timely DV. Language of the letter isn't important. The statute, and no caselaw that I know of, in any way indicates specific language you the consumer must use. It could be in crayon, lined paper, and state "This ain't mine" along with your name and address, and I suspect that would constitute a DV letter. You don't even have to sign it. If you sent it CMRRR, and saved the paper trail, you DVed and you can prove it in court. If the very next thing mailed to you wasn't proof that the debt is yours, then they are in violation of FCDPA. Every call, letter, lawsuit, and CRA reporting to all 3 CRAs is a violation.
Fred Hanna has a special place reserved in H*ll. In response to EVERY statement you get from these twits regarding this claim, send a DV letter. Some CAs are slow fish and you have to hit them repeatedly. Repeat yourself. "I sent a letter requesting validation of this debt on December XX, 2005. See enclosed copy. This debt is not mine, you have not validated, and you are now in violation of FCDPA."
Every time they switch it to another CA, send them a DV letter. That's a game played when they know the debt isn't your. Switch to another CA and make you think it's a different debt. If that CA has previously contacted you and you DVed them, advise them they are now in violation.
If they sued or are threatening to sue, I suggest you take all your paperwork and contact a consumer lawyer. Try myfaircredit.com or naca.net
Call and inquire about discussing your situation. You might get a lawyer, more likely a paralegal, perhaps only a receptionist.