If the mystery dialer is just fishing to see when somebody picks up, you will probably get a follow up call by a live human trying to sell you gawd-knows-what.
When an actual human calls you, and as soon as they start their spiel, interrupt them and tell them in an irritated voice ‘THIS is a multiplexed data communications T1 line and you are interfering with federally regulated voice data transmittals, remove this number from your list immediately!’
Most of the time, you’ll get a sheepish ‘oh, I’m sorry’ and they won’t call back.
:)
I like that. I used to answer some of them, and just say "Hello?.... Hello?...."(act peeved) "Your nickel, start talking..."... "Hello?"
Ignoring anything they said (sometimes funny, often obscene). Eventually they would hang up. No sales pitch.
Now, if they don't leave a message, they do not get through.