USCF Official Rules of Chess P.93

20N. Electronic Communication Devices.
"Players are not allowed to leave the playing venue without permission from the arbiter... "Without the permission of the TD, a player is forbidden to have a mobile phone or other electronic means of communication in the playing venue, unless they are completely switched off." - 1. the first offense includes a stern warning and reminder of the rules. 2. the second offense levies a time penalty of 10 minutes or half the time remaining, which ever is less. 3. the third offense will be forfeiture of game. (these penalties are actually lighter than those suggested by the USCF rule book). Note: If a player is expecting a possible "emergency" call because of family, business or health reasons, the TD may allow him to place his phone on vibrate. If it vibrates, the player is required to immediately retreat to a private area before responding to the call. -- Otherwise, all cell phones must be "off" during tournament competition. NMLF