PURPOSE: To call terminal stations equally without reference to the intensity of radio waves from the terminal stations to a master station by receiving a free- line signal from the master station at a terminal station, and then changing the timing of transmission for call signal after a signal break is detected.

CONSTITUTION: A master station sends a certain-time free-line signal (a) to a free line periodically. Once, receiving the free-line signal (a), a terminal station stops a scan at a current channel to detect a break (a) of the free-line signal, and then sends call signals (b1∼b3) certain times (t1∼t3) after the free-line signal break. Said certain times (t1∼t3) are changed at random or cycle on single-time calling successively in the order of t1, t2, t3...tl.