You are referring to the FDCPA. This act applies to anyone attempting to collect the debt, as long as the debt did not originate with that entity.
If you seek validation in a written request, a copy of the validation letter is required to be provided to subsequent collectors or debt buyers, as long as the transfer occurs after receipt of the letter.
You should review the FDCPA.
I am not sure whether that Circuit has precedent cases dealing with the FDCPA, that have held that all others who subsequently work the alleged debt are bound and deemed to have been put on notice from that point forward. Personally I doubt that would be the case, unless it could be proven that all who worked the alleged debt thereafter were given knowledge of the validation request that had not yet been answered. If you're having problems with a collector, feel free to email me your info or contact my office Monday and we will try to put you in touch with an attorney licensed in the federal court for your federal district in North Carolina who can help you at no cost out of your pocket.
Please do not take my answer to be legal advice that would establish any attorney-client relationship. Please take it as a general response from my own experience in response to your question. I hope you find it helpful.