In the past two days alone, I've gotten over 20 spam calls with none of them leaving a message. Just 3/4 seconds of silence on my voice mail. One call the other day left a 22 second blank message.
For those on iOS you don’t need an app to do this.
You can go to settings -> apps -> phone -> silence unknown callers.
This will send them straight to voicemail and the call will show in the recent calls list.