Scammers or such. If you call back anonymously (block your #!) you get a fake sounding message about how the call can't be connected/number is no good.
My guess? You call back = they harvest your number as valid. Either that, or it really is a bogus number and they are violating federal law by spoofing their number on the caller ID with a fake one. Either way, not somebody you should waste your time on. Don't call back and don't pick up.