My own policy is to either turn my phone off or on vibrate. Many times I just leave it at home or in the car. After all, your guest is who you came to spend time with and talk to was it not?
I would take offense if I took time to be with you and you sat and talked on the phone the entire time we were together, not to mention the people around you don't really enjoy hearing your phone ring playing the first verse of Jingle Bells at ear-splitting volume while you poke around your purse looking for your phone.
Long before we even had cell phones we had voice mail. Unless mom's in the ER on life support there is no reason why you can't just silence it and let whomever leave a voice mail.
Wake up folks! It's time to talk to the person with you rather than chattering on the phone with others who just have nothing to do.