I'd vote that it's a scam, and to ignore them. There may not even be any debt owed by your wife, and they're hoping that you're disorganized and that their phone call will scare you into paying something.
Legitimate debt collection companies should be able to give you details about the debt they are trying to collect on. These details, such as the original creditor and original amount owed should be enough for your wife to ascertain whether or not the debt is valid.
They also should follow the debt collection laws. You can google these and find out what rules they must follow. Scam companies generally won't follow these rules; most legitimate firms will, although some may burn the edges of the law.
Scam companies will try to get you to pay with gift cards and money orders, and will be quite insistent that your wife may be thrown in jail or that the sheriff is on their way to arrest her right now. All of that is balderdash; people are not arrested or imprisoned for failing to pay debts. Legitimate companies should not (and generally do not) make those kinds of threats.
Even if the debt is valid, you may want to carefully consider whether or not and when to make payments. I think there is a rule that after seven years of inactivity, a debt becomes uncollectable; making any sort of payment restarts that seven year clock. I'd be especially careful on any old debt of your wife's that her last payment was 6 1/2 years ago or so. (There is also the moral aspect of paying your debts even if they are uncollectable; I'll leave that topic alone.)
If there is any legitimate debt, it should also show on at least one of your wife's credit reports. If you had all three agencies pulled recently and there are no debts of this sort, then I'd say that makes it even more likely it's a scam.
