What happens if your phone starts ringing in a tournament??

If it's that important you should

a. arrange to have it at some other time

b. not participate in the tournament.

Cell phones should be off and left in your car or at home as the noise and jumping up to leave the room is a distraction to everyone around you, quite apart from the suspicion of cheating.

For many officially sanctioned tournaments, having your phone go off can have penalties including but not limited to:

1.) deducting time from the offender's clock

2.) offender forfeits their game

3.) offender withdrawn from tournament

4.) offender banned from future tournaments

Depending on how official the tournament is, the organizer may choose stricter or kinder penalties.

There are many options if you think that you may be contacted during a tournament.  You may:

1.) Have a friend or family member nearby to keep your phone and answer any phone calls you receive.

2.) Give out the phone number of the front desk of where the tournament is held.  They most likely will have instructions on how to find the TD to deliver any important messages.

3.) Keep your phone off or in silent mode (vibrate-only mode may still be a distraction) and check it between rounds away from the playing hall.

If your phone goes off in a tournament I will throw a piece at your head.

Edit to add a potential penalty:

5.) Your opponent or another nearby player may throw a piece at your head. (Thanks TheHumanOwl)

Seriously, though, having a phone go off is a major distraction to everyone in the playing hall.  Hence the often harsh penalties.