To attain a camp-on function at a reduced cost.

This method is configured such that a caller communication device 10 starts calling to a called communication device 18 and the caller communication device 10 transmits a 1st notice signal to the called communication device 18 in response to the detection that the called communication device 18 cannot receive the call from the caller communication device 10, and the 1st notice signal automatically triggers a 2nd notice signal from the called communication device 18 to the caller communication device 10 when the called communication device 18 reaches a state of being able to receive a call from the caller communication device 10. The caller communication device 10 transmits a 1st notice signal including the network address of the caller communication device 10, and after caller communication device 10 receives the 2nd notice signal from the called communication device 18, the caller communication device 10 starts the calling again to the called communication device 18. Thus, by employing the inter-device signal exchange in this way, a camp-on function is provided.