I believe Rule 3 section 3 addresses it...... (from '06-07)
| Art. 1... A substitute who desires to enter shall report to the scorer, giving his/her number. |
| a. | Between quarters, at halftime and during a time-out, the substitute must report or be in position to report to the scorer, prior to the warning signal which is sounded 15 seconds before the end of the intermission for a 60-second time-out and prior to the 20-second warning during a 30-second time-out. |