I can understand how someone with my coworker's voice might lull myself into a false sense of urgency and safety.

To the point of sharing an OTP code over the phone from a strange number? I'm sorry, no.

You could give them a false number and see what happens. That might trip up their script enough to reveal they aren't who they seem to be. Just play dumb -- "I can't understand why it's not working, that's the number on my phone...."

That's also a good idea.

They could trivially spoof the number they're calling from to match.

Or I could call them on a known point of contact, like a phone number or IM.