Some of these robo-dialers will hang up right after the fourth ring (that they hear) because voice mail and answering machines usually begin after the fourth ring unless programmed otherwise. The caller does not want to waste time leaving any kind of message on an answering machine when the the equipment could be calling another sucker.
So I try to get to the phone before that fourth ring ...
My method : I've been waiting three rings, picking up the phone, and then saying nothing.
After 5 seconds or less, I hear click ... click ... and a hangup.