NEVER, NEVER, NEVER give any personal information, especially your SSN, to someone on the phone.
It's SBCS Debt Collections. They robodial and may not have any idea if you the presumed debtor or not. That's why they try to get information from you. Here's a ripoff report in which they had the wrong person so scared that he actually gave them money. That's the kind of "people" you're dealing with:
http://www.ripoffreport.com/r/SBCS-Debt-Collections-And-Phone-Company/nationwide/SBCS-Debt-Collections-And-Phone-Company-Ripped-me-off-Classic-example-of-a-ripoff-mental-220671
Read here:
Your rights (what a debt collector can and cannot do).
http://www.consumer.ftc.gov/articles/0149-debt-collection
How to tell a real from a fake debt collector:
https://www.consumer.ftc.gov/articles/0258-fake-debt-collectors