I have to reply as a cell phone user.
First of all I never leave a cell phone on at church, weddings, special events, seminars, the theatre, etc. I wouldn't dream of doing so on a date! I think that any of the above is considerably rude.
But at any other time -- the cell phone is on. This is due to my type of business -- in this day of instant gratification, if someone can not reach you for a reservation, they go on to the next. People have a tendancy to call at meal times. My operationis not big enough to have a manned front desk 24/7 beyond myself. So if I want a dinner prepared by someone other than me, or if I want to go to the store, I will be connected. Cell phone drivers need to pull to the side of the road to conduct business!
I am also considerate enough that I leave the table and go to a powder room (if they have one sepate from the stall area or outside to conduct business. I don't stay at the table and disrupt my fellow diners. And yes the volume on the ringer is turned down (my next phone will have vibrating capabilities). I do feel that a manager, FOH person does have the right to ask a loud individual to take their business outside if they do need to take the call.
Just a note from the other side,
Lynne