Recalls are race committee actions, and the boats still have to know their own position.
29.1 Individual Recall
If any boat is over at her start, or must comply with the I flag rule, the committee promptly shows X with one sound and keeps it up until all such boats have returned, or for four minutes after the start, or until a minute before the next start, whichever is first. A boat that is over must return and start properly; failing that she is scored OCS. If U or black flag is in force, X is not used.
29.2 General Recall
If the committee cannot identify the boats over the line, or the procedure went wrong, it may recall the whole class with the First Substitute and two sounds. The new warning signal comes one minute after the First Substitute comes down.
Common misreadings
"No hail, so I was not over"
The rules require a flag, not a hail. Many committees hail as a courtesy; failure to hail is not a ground for redress unless the sailing instructions promise one.