AT&T - Cellular Connections AT&T Debt Collector using bullying tactics Internet

I am being harrassed by a debt collector for unknown reasons.  They have called at least three times a day this whole week.  When I spoke to them, they insisted that i give them my social security number to verify who I am -  I refused to furnish the info, however I agreed to verify the info they have but they said they have noted the account that I refuse to cooperate. 

 They will not even give me the name of the "so-called" unpaid debt.  Today the gal told me again that since I won't give them my private information that they will continue to call me everyday until i cooperate.

 These people are nasty...