[US] possible debt collector callS, real or scam?

A user received multiple calls from a number listed as 'Merchants and Medical Collection,' which they suspect could be a scam or legitimate debt collector. The number has a local 810 area code, differing from the agency's official 1-800 numbers, and left a silent voicemail. The user has unpaid debt from two years ago but has not received any mail despite moving twice with forwarding.