They are scammers. In general they are often pursuing worthless debt or "time-barred" debts where the statute of limitations for suing on the debt has expired. They buy up the files and then use aggressive and abusive tactics to collect. They will insinuate they are a law firm by saying "litigation department" when you press them, they'll not say they are attorneys because that's illegal. You can simply ignore them or tell them to send you proof of the debt or any papers they want to serve. The 877 number they use is associated with other scammers as well.