Not always true. My landlord recently had a contractor call me. I did my usual "pick up and don't say anything" routine for unrecognized numbers, and the contractor silently hung up and never called back. Thankfully my roommate actually answered the call, but pick-up-shut-up prevents legit people from leaving voicemails and sometimes prevents legit people from reaching you entirely.
Personally, I would utter a confused "hello?" if I was calling somone, the ringing stopped, and no one said anything, but I guess not everyone would.
I could easily see someone like a contractor calling from the road or otherwise not paying full attention to their phone. They likely never realized you answered and needed the "hello" to refocus their attention.
As with 'craftkiller, I've noticed that I do need to make some kind of noise. I've settled on subtle light coughs or grunts (nothing anyone would think twice about, but which will definitely trigger a "oh this is a human!"). I figure it might still fool some percentage of automated systems which detect whether a human (and which human) is actually there or not based on automated transcription.
