a "software-generated voicemail message"
That's interesting, but if the Boss is calling you, you have his number and can check that it is the right one. Plus, your boss may a have croaky voice, but I don't think software is yet up to the stage where it can convincingly impersonate someone you know.
Of course, I'm situating the whole thing as boss-calls-financial-person-as-usual kind of scenario. I don't see how this could really work. If the company is small, the finance guy is going to know the boss very well and it won't pass mustard. If the company is big, the finance guy being called will wonder why the hell he's the one called, go to his manager and it shouldn't pass mustard either.
And yet some numbskull did fall for this.
There's always a better idiot.