While we are all called upon to "multi task" in our day to day lives, the real truth is that when you multi task you don't give your full attention to anything and run an increased risk of making bad judgment calls or making mistakes.  I am a firm believer of keeping meetings as short as possible and following an agenda so that the meeting stays on track.  Therefore, I feel that cell phones should be turned off - not just silenced - during meetings of 30 minutes or less.  That way you avoid distractions and have everyone's undivided attention.  This should make the meetings more productive and will help keep them short!  All that said - it is a good idea to have a contact for emergencies during such times.