Key phrases make sense to put in place, but another easy safeguard is:
"Before you send anything to anyone, ever, call them back. Doesn't matter if it's me, the bank, a lawyer, whatever... tell them 'hang on I have another call coming in, let me just call you back in a few minutes, okay?'"
"They say they're going to cut a finger off every time you hang up."
Not a thorough safeguard, if scammers have half a brain cell they can provision a VOIP number for such a request. They’re nothing if not accommodating.
I overheard a cab driver being scammed by someone claiming to be HMRC (UK version of the IRS), and when he asked to call them back they managed to convince him to call them back on Viber, and he was about to when I intervened and pointed out to him that 1) what I'd overheard was blatantly a scam, 2) if he was unsure, to go to their official website and find their number...
If you're going to get people to call you back, it has the problem of ensuring that they call you back on your real number - giving reasons why they have to call you back on some other number is way too easy ("I've lost my phone", "my phone is at home", and so on)