A debt from 1999 to a business has probably long since been written off and can't be claimed again. A personal debt of $400 your husband would not forget no matter how bad his memory.
Debt collection agencies are evil things but even the most aggressive ones won't reveal information to anyone but someone they believe has a right to know, specifically the debtor or a spouse or other relative.
When I moved I got a lot of calls from debt collectors for a guy that apparently used to own a construction company. When I actually got them on the phone (instead of their hostile messages) and asked "what the $*%&$# is this all about" they wouldn't tell me. So I told them a few things requiring more $*%&## special characters, then turned off my ringer and my answering machine (everyone that I want to talk to, and even some I don't, have my cell #).
I'm going to concur this very well may be a scam. If so, extortion is quite illegal. Get all the info you can on the company and/or persons contacting you. Keep asking them for information until they hang up on you. Street address, if they are a publicly traded company, CEO's name, etc, etc. It feels really good when they hang up on you. And they will. Small victories :-)
"I have heard there are troubles of more than one kind. Some come from ahead and some come from behind. But I've bought a big bat. I'm all ready you see. Now my troubles are going to have troubles with me!" - Dr. Seuss