[Wiser now]

Clark Howard has information about this kind of “debt collection” at his website. The “debt collection” company settles with the provider, in this case Verizon for whatever they can get then attempts to collect the debt from you for whatever they can get.
You send them what Clark calls a “drop dead” letter.
Tell them you do not owe the debt they allege you owe (important wording) and that under the Fair Credit Reporting Act, they may not put anything on your credit report. You then keep a copy of the letter forever.
www.clarkhoward.com

[BearCub]

Tell them you do not owe the debt they allege you owe (important wording) and that under the Fair Credit Reporting Act, they may not put anything on your credit report. You then keep a copy of the letter forever.

Clark Howard and his followers know very little about the reality of dealing with debt collectors. Clark himself isn't too bad but the people on his message boards really suck.

If it is within the 7-1/2 year reporting period they absolutely MAY list the debt on your credit report even if you dispute the debt. In that case they must ensure the account is marked as disputed on your credit report.

If you ask for validation and they haven't yet reported it to the credit reporting agencies, they CANNOT report it until they provide the validation. That is because the mere listing of the account is considered collection activity, which must cease until they validate the debt.

The whole thing is pretty complex, but in this case isn't. Send the "do not contact" letter an be done with it.