[APG]

Here's the entire rule from the 2013 Rule Book:

Rule 5, Section 3

Article 3 Each offensive and defensive team is permitted no more than one player on the field with a speaker in his helmet. Each team is permitted to have a maximum of three active radio receivers to be used on offense by its quarterbacks, and a maximum of two active radio receivers to be used on defense by players who have been designated as a primary and backup user.

Clubs that have a player whose principal position is as a non-quarterback (e.g., wide receiver, running back) and who also is used as a quarterback from time to time must have two helmets for that player—one with and one without radio components.

When a quarterback enters the game for the first time, or re-enters the game if he has previously been in the game and removed, he must report to the Referee.

Whenever the backup defensive user enters or re-enters the game wearing a helmet with a speaker, he must report to the Umpire. If the primary defensive user subsequently re-enters the game wearing a helmet with a speaker, he must report to the Umpire. If the primary and backup players have been removed from the game, no other player may wear the radio speaker in his helmet. Teams must use other methods to communicate signals to their players.

It is not necessary that the offensive player with the speaker in his helmet receive the snap.

For special teams’ plays, only one player per team with a receiver will be permitted on the field.

The coach-to-player system is not subject to the equity rule. In the event one club experiences a coach-to-player radio system malfunction or failure, the other club does not have to shut down its system and may continue using it. However, if the coaches’ intercom system has been completely shut down on both sidelines pursuant to the equity rule, all coaches’ headsets must be removed, and radio communications from the sidelines to the field must be conducted by walkie-talkie only.

All players that have radio components in their helmet must have a decal, supplied by the League, displayed on the midline of the rear of the helmet. Players who have speakers in their helmets must be identified in the Communication System section of the Game Day Administration Report that is submitted to the Referee or a member of his crew no later than one hour and 30 minutes prior to kickoff.

For additional requirements and policies pertaining to the system, including regulations for the storage of a backup helmet for a non-quarterback who is used as a quarterback, or for the backup defensive player’s helmet, please refer to the Policy Manual for Member Clubs/Game Operations.

Penalty: If a player fails to notify the Referee of a change in his status when required: Loss of five yards for illegal substitution.