Fortunately, we rarely have a problem with guests bringing a phone to the table. But there was one guy who brought THREE to the table, baseball hat on backwards, leg jiggling under the table....you know the type. I felt really bad for his lady friend who mentioned that she had hoped the B+B stay would help him to unwind and relax. He, on the other hand, was as happy as a clam...taking calls and scheduling work....international insurance company guy...handled all sorts of dire, international disasters. Blech!
He was an exception...so no signs needed at our table.
most of ours bring them to the table but I insist on silent or calls to be taken in the lounge as its rude. We mainly have business travellers and they sit on their own table (ie 1 on each table) so they can't do what they like as long as they are quiet