During the O2C Doeberl Cup there were a couple of "Mobile Phone" incidents. Before the start of each round the players and spectators were instructed to switch off their mobile phones, and the penalties for allowing their phones to ring were spelt out.
The penalty for a player was immediate loss of the game. This only occurred once, and was accompanied by the look of surprise on the offending players face, followed by a "Don't I get a warning" query. Sure, but the warning came before the round started. To the players credit he later spoke to one of the arbiters and accepted that he had broken the rules.
In the case of spectators, the penalty was to be excluded from the playing hall for the rest of the day. The question asked was "What happens to players who have finished their games, but break the rule?" The short answer is that they are treated as spectators and are excluded from the playing hall. However if they have a second game to play, they are allowed back into the playing hall to play the next game.
The other incident involved a player who had finished his game and had left the hall. At some point he switched his phone back on, and as he was re-entering the hall, his phone began to ring and he immediately left the hall. Although I didn't witness the incident myself, in discussion with the other arbiters it was clear he was only at the doorway to the hall, well away from any games in progress.
As no one appeared to be disturbed by the ringing of the phone, the decision was to give the player a stern talking to, with some finger wagging on my part, and a warning that "this better not happen again".
Ultimately the issue was resolved with the required amount of common sense, and no one went away feeling that the process treated them unfairly. And as an arbiter, this is something I will always try and aim for.