I get several voicemails a week from unrecognized numbers that are ~3-5 seconds of silence. I suspect it's the spammers' autodialers not recognizing that there'

I get several voicemails a week from unrecognized numbers that are ~3-5 seconds of silence. I suspect it's the spammers' autodialers not recognizing that there's not an actual person on the line, and giving up once there's a few seconds of silence (post "leave your message at the beep" beep).