...I got one of these calls this afternoon, and when I called back so that I could ask to be taken off of their harassment lists, it was an unknown number. So, through reversephonelookup, I found the following:
Area Code (NPA):    765    Exchange (NXX):    601
Original Service Provider:    U.S. CELLULAR    Zip Code Coverage:    46050, 46057, 46067
Original Service Type:    Wireless    Carrier Type:    Wireless Provider
Original Coverage Area:    Frankfort, IN    Switch CLLI Code:    LGPTINCX1MD,
so. it turns out to be one of those throwaway pay-as-you-go cell phones.... scam!

left me a voicemail "receive a free $1500 security system" . . . press "1" for something or "2" for something else . . .

Its interesting how US Cellular keeps telling all of us the number is no longer in service – and yet – we all keep getting new calls from this same scammer from exact same number. US Cellular is lying about its business relationship with this scam security company.  File a complaint with the FCC. Make sure your complaint includes the carrier service (US Cellular) and its active participation in a known scam.    


Area Code (NPA):    765    Exchange (NXX):    601
Original Service Provider:    U.S. CELLULAR    Zip Code Coverage:    46050, 46057, 46067
Original Service Type:    Wireless    Carrier Type:    Wireless Provider
Original Coverage Area:    Frankfort, IN    Switch CLLI Code:    LGPTINCX1MD,

Because the number is faked, he could use your number as his CID.

They left a message. Trying to sell me a security system. I already have one.