The other day I was having a bad day and had a call from an unknown number, so I decided to pick up but let them talk first. They just hang up and have not called again.
I don't think it was a good idea to pick up in the first place, but it was curious in the end.
Iirc, that's usually a sign that the robot caller has found a machine on the other end, since most people first respond with some sort of greeting right off the bat.
Glad to hear this as a person who will not talk first for unknown numbers. Sometimes unknown calls are legit—like from the office of a doctor I was referred to, that I was too lazy to take the number of—so I always pick up. I just also stay silent and let them prove themselves human.