Spoofed number that is not currently assigned to a user - and those are easy to find by dialing. This phony call center is somewhere in India and they employ people who absolutely KNOW they are scamming people. Just think for a moment, why would a legitimate duct cleaning business need to lie about a phone number, and there isn't any biz like that which could handle the flood of clients that a legit telemarketer would generate.
Sadly, the DO NOT CALL list and regulations do not worry to this scammer, as they are not in North America and are untouchable by our laws.