I've done this - but only when a booking has gone a few minutues over the allocated time, and the client is overstaying his welcome. It's a good way of clearing someone out quickly if they haven't got your previous hints when it is time to go. I don't care if they think I'm being rude as when their actual booked time is fininshed, I should be free to take other potential appointments, and I'm not losing out because someone is taking the mickey. I'm very polite at first, but some guys blatantly pretend to lose track of time.
Other than this, phone is on silent or off. If I get a missed call during a booking, I just send a text saying "Sorry I missed your call, but I am free to talk now".