NFL quarterbacks are allowed to have receivers and speakers in their helmets but no microphones. One defensive player on the field is allowed to have the same set-up.
The rules allow three quarterbacks on each team to have the system in place. One coach who is on the field communicates with the player. On defensive squads, only two players per team are allowed to have the devices. Both are not permitted to be on the field at the same time. The equipment is not foolproof, however, and interference is possible. Sometimes players hear conversations among concession stand employees or other people in the stadium. On occasion, voices inside helmets have originated from completely outside the stadium.