Even if the debt were valid (and you know it isn't), there isn't a thing they could do to you.
Assert your FDCPA rights. It's against the law for a collector to sue you or threaten to sue you on a time-barred debt. If you think a collector has broken the law, file a complaint with the FTC and your state Attorney General, and consider talking to an attorney about bringing your own private action against the collector for violating the FDCPA.
Another one of those "unknown numbers" that we do not answer. It has just started recently, and they seem to call multiple times a day. Another number from the same area code also calls shortly after this number. Wonder if they are "ganging up on us" or if they think we're so stupid we'll ignore one, then answer another! Doesn't matter, we do "ignore" quite well!