IMO, it would be unspeakably rude to essentially tell guests "we don't trust you to behave like adults so we're going to ban your phone usage".
I can understand the irritation with phones, but, really, requests can be politely made, orders cannot be politely given.
(and there have been times in the last few years where a contact number wouldn't have cut it--I was the contact person for 2 relatives in hospice care, i was the 'emergency contact' for family that was travelling in a relatively risky overseas location, etc--and the important people knew I was reachable 24-7 for these purposes. Contact everyone and saying "oh, everyone please know that next Saturday between 8 am and 2 pm, you should reach me at this number instead" would have been an hassle of epic proportions.
People handled these things before cell phones but, honestly, I've currently planned my life around having the phone on and available--I'd have done my planning differently if the phone wasn't available.)