I seem to be getting a flood of calls of this type on my cell phone - scamming, robocalls - from all over the country! Yikes, what's going on? Didn't answer any of those calls, and don't plan to.
One short ring then hung up. I called back, but was told all circuits were busy. Called again to hear a busy tone. My guess is maybe a computer programmed to register existing phone numbers by calling, then ends the call once it realizes the line is either active or not. Just a hunch though.