Well, since I turned out to have guessed wrong about there being room for a Sweets & Savories-specific phone manners discussion on the S&S thread (not that anybody necessarily had anything more to contribute to it), I'll talk about a couple of general pet peeves in the department.
1) I mildly dislike being put into a "decision tree" (it that's the term) of automated prompts. ("For a reservation for today, press 1. For a reservation any other day, press 2.") I don't mind this when I know I'm calling at other than business hours, but it's off-putting when you know someone's there. Honestly, the same person can't just pick up the phone for people making reservations for tomorrow vs. today?
2) I really hate when a person picks up and speed-talks, "HelloRestaurantXCanYouHold?"click. I think it's the tone of frantic, overworked, overwhelmed desperation that irks. It suggests that when I get to your restaurant, I'm going to find the rest of the staff equally unable to manage the amount of business you have. (And if you're going to ask if I can hold, at least give me the space to get in a one-syllable answer before you put me on hold.)