Nelson Watson & Associates - Pinnacle Credit Services SIAD I OWED ON A PHONE I ALREADY PAID OFF!! Haverhill Massachusetts

I recieved a letter from Nelson, Watson & Associates saying I owed another creditor, Pinnalce Credit Services, a debt of $327.11 for a Verizon account that I paid off last year!
I accepted a settlement with Verizon last year and paid them off.
Then out of the blue a year later I recieve this letter.
Nelson LLC., is telling me that it's for another account I had with Verizon ,and when I informed them that I never had more than one account with Verizon, they claimed it was debt I owed on another line because I had one account with 2 lines. Why would Verizon offer me a settlement on only a portion of the debt that I owed and not the whole amount?!
Verizon has absolutley no more records of my old account with them because it was settled and paid.
The letter requests that I pay Nelson and Associates, not Pinnalce.
What is Pinnacle? Is it a collection agency or a scam company that buys old debt information and then tries to get other companies to collect on it ?
If so, how does a legitimate agency, which Nelson and Associates so far appears to be, fall for these scams?

Vicki
longwood, Florida
U.S.A.