Bare minimum to me should be as follows:
Home calls; called from a listed number from the same address as the call location. Both I will search via google.
Hotel call; called from the room phone leaving name and room number. I always call back via the hotel phone anyways.
I always perfer hotel calls and if I am not 100% satisfied on a home call that I will be safe....Forget it.
I would've been telling them no, and help them find the person all for a nice tip.
In the past when they have run those contests it amazes me that they come up, and still ask you the question aften they just saw you say no the the last person.