Only the players involved may call a foul. In the event of a foul call, the opposing player receives cue ball in hand anywhere on the table. This means that the player can place the cue ball anywhere on the table (the cue ball does not have to be behind the headstring except on opening break or immediately after a foul on the break).
A player must stop shooting when a foul is called. If a player refuses to yield to a foul call, the opposing captain may protest.