PURPOSE: To detect the status of a radio channel without using any control channel by making slave cordless telephone sets other than the slave cordless telephone set being used for communication to monitor the speech channel by intermittently receiving signals from the speech channel.

CONSTITUTION: Plural slave cordless telephone sets 301-30n are provided as extension telephone sets and a radio channel is formed between the slave telephone sets 301-30n and the radio transmission-reception circuits 13 and 14 of a master telephone set 10. When one of the slave telephone sets 301-30n is set to a talking state, the other slave telephone sets are set to intermittently receive signals from the speech channel. When the received signals are voices, it can be recognized that the speech channel is busy. When the channel using status is changed from the talking state to a holding state, terminated state, etc., the master telephone set 10 sends information to the slave telephone sets which make the intermittent reception to indicate the changed state. Therefore, each slave cordless telephone set can recognize the used state of the speech channel by detecting the information sent from the master telephone set 10 through the speech channel.