Don't let the caller worry you - it's a scammer trying to scare you into paying a bogus debt.  Report the calls to the FTC, the FBI and your state's Attorney General.  Next time this joker calls you, tell him you're recording all calls he makes to give to your attorney (even if you don't have one) for a lawsuit for multiple violations of the FDCPA.  Remember - no one calls you ahead of time to tell you that you'll be served.  If a caller refuses to mail (not email or fax) a debt validation letter proving you owe a debt to them, then you owe them nothing and can treat them as you would any scammer.

Telling me i owe from a credit card opened back in 1990. THIS IS A SCAM! That i have to settle this by the following day at noon. It was for thousands of dollars. which i never had any credit card over a few hundred dollars in my lifetime. The man made me nervous & said i will be serve with a summons & i would have to appear in court if i don't settle this. i ran my credit report, just to see & nothing of that sort was on it!!