Take it one step farther. Robo callers hang up on 4 or less rings 99% of the time. Set your voicemail to pick up on 5 rings. This works automatically and you don't have to be home to manually pull the plug. Since your machine didn't connect, your # has not been validated as working.
Caller ID showed as "Unknown" so I didn't answer and then my messaging/recorder kicked in and for the next 6.5 mnutes there was total silence recorded so clearly the line was still open. Robo-idyit!
Called tonight after 9pm. Answered because telemarketers & scammers know not to call so late; so must be someone I know. Wrong. I said "Hello" a few times. No answer, just silence.
I called the number back and the recording said if I want to be removed from their preferred marketing list, press 1. Of course, I wasted no time in doing so. The recording then said please allow up to 72 hours to be removed from their list.