I am getting the same thing - about every hour I get two or three calls back to back from the same numbers below.  I think it's a scammer who is spoofing their numbers.  If I don't recognize the number I let it roll to voice mail.  No message means I won't return their call...

Phone calls. No message left when I don't answer. Happens several times a week.

They keep calling, but never leave a message. For the past week, I've been getting calls all day long from different various numbers all over the country: 415-992-7492, 917-720-3087, 310-598-2198, 310-598-3414, (310) 773-3861, (415) 922-7692 and so on....I have about 30 missed calls from there various numbers.  When I call them back, nobody answers OR I get a fast busy signal.  From no on, when they call, I press and hold a number key so they can listen to a nice loud beep in their ear.  Losers!