Never put the phone down. Let the other person put it down instead. Only really works with mobiles.
I’ve been on calls when the other caller assumes that at the end of the conversation I have rung off. But I haven't and I'm still there, still listening.
It’s a great way to hear what they really think, especially if there is someone else with them for them to talk to.
Be warned though, you might not like what you hear and it’s embarrassing if you get caught.