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And even if you can easily think of a scenario, would there not be something ethically questionable about doing so, if the person at the other end had no idea who or what they were talking to?

Generally I consider something ethically questionable when a person is harmed in some way. Can someone explain how a receptionist might be harmed by unknowingly booking an appointment with a robot instead of a person? The real person shows up. The business gets an appointment. The receptionist does their job. Where does the ethical question come in?

[Hydraulix989]

Deception is a form of harm.

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So children are harmed by believing in Santa Claus? If you politely tell someone you enjoyed their cooking even if it wasn't to your taste, you've harmed them?

Context matters. Deception, in and of itself, is not inherently harmful.

It makes literally 0 difference in the life of the person booking the appointment if it's a robot or a human telling them the desired time and date.