My pet peeve is not necessarily how people behave ON the phone, but rather when they use the phone. When I have meetings with people, my desk phone goes into "do not disturb" mode, and my cell goes to vibrate and is NOT answered while I'm having a face to face meeting with someone.
The number of people I meet with nowadays that stop a face to face meeting to answer a phone call is astounding. Working in the emergency response arena, I cut slack for someone who may need to check caller ID or quickly answer a call in case it is an emergency, but if it's not an emergency, they should respond with a, "I'll call you back", not leave me cooling my heels while they take down a list of groceries they need to get on their way home.
Oh yeah, also people that constantly disrupt multi-participant meetings by not putting their cell on vibrate or "off" and getting up every ten minutes to answer various calls. Either attend the meeting or answer your phone -- make up your mind (and I HATE meetings, but if we all have to sit through it, please -- common courtesy).