Received a call from the "general investigations unit" at this number.  An "agent" said to call back as it would only take 5 minutes of my time and left a reference number to use on the call back.

This is a debt collector.  If you receive a call from this number, save the voicemail and report it to the state attorney generals office.  Not only is this type of call a civil crime, it's a criminal one too.

Also, check your states statute of limitations on debt.  If you owe a past debt, check and save your credit report regularly to ensure that they cannot claim you've made a payment to reset the collection clock and also ensure that the debt comes off in 7 years.  Not everyone who cannot pay is a dead beat, circumstances can make some bills impossible to pay at any given time and people like this are not allowed to harass, threaten, intimidate or mislead you.