Unless someone is calling to tell you that you have been given a transplant organ, are on call for a job, or the babysitter is calling then I think it's incredibly rude to use the phone at the dinner table. In the case of the job call, organ being ready, or the babysitter calling then I think a quick "this is an emergency, I really have to take this one guys" is OK, but after saying this you then excuse yourself and leave the table!
I would probably alert my companions if they were good friends that I was on call for the night or that maybe it would be my lucky night and the transplant might happen. A nice preemptive way of letting people know I do not intentionally plan on being a an ill-bred person when I answer my phone.
The phone really needs to be off and tucked away in a purse if this is at all possible, this is what I do. When I am out with friends, I am out with my friends, NOT my friends and my phone. Luckily my friends feel the same way unless there is some big deal they had to stay in contact about.
People understand that things come up and emergencies happen, but many do not understand why at dinner out you need to gabber on the phone about work gossip or other trivia.