Conventionally, there is known a mobile phone with a group call function to make a call to a plurality of persons in a half-duplex communication system called Push To Talk (hereinafter, referred to as PTT). In a group call to be made by such a group calling function, when the send button in a mobile phone is pressed, the floor is granted to the mobile phone. While the send button is pressed and held, sound can be sent to other phones.
When a user to make a group call performs a calling operation designating the phone numbers of the plurality of communication partners, the phones designated by the phone numbers is called via a server managing the call. When the called phones make a reply to the incoming call, the group call is enabled among all of the mobile phone having performed calling operation and the phones having replied to the call.
The group call function is assumed to be used to transfer information unilaterally from a person at the calling side to a plurality of persons at the called side, such as in the situation in which an operation supervisor provides working instructions to a plurality of workers. In such a usage mode, it is inconvenient that the persons at the called side need to make a reply each time the persons get an incoming call. Accordingly, for the foregoing group call function, there may be a mechanism that allows a reply to be automatically made from the phones at the called side to the incoming call without requiring the persons at the called side to perform a reply operation, unlike when making a reply to an incoming call at a general phone.