Patent ID: 8792927

Claim:
A mobile station which (i) monitors, in a monitor cycle, a paging channel slot which stores a message therein and is transmitted by a radio base station in a transmission cycle, and (ii) performs a process according to the message, comprising: a PTT setting reception unit operable to receive a PTT setting operation from a user for setting a PTT (Push to Talk) mode, in which a voice packet communication is available; a sign-in unit operable to transmit an allocation request to the radio base station when the PTT setting reception unit receives the PTT setting operation, the allocation request being for allocating, to the mobile station, an IP address which is required for the voice packet communication; a monitor cycle switching unit operable to switch, when the PTT setting reception unit receives the PTT setting operation, the monitor cycle to a predetermined short cycle which is shorter than a cycle at a time of receiving the PTT setting operation; a voice reproduction unit operable to receive a voice packet which is transmitted from the radio base station after a PTT CALL is detected in the message, and reproduce a voice based on the voice packet, the PTT CALL being a request for the voice packet communication; a PTT cancel reception unit operable to receive a PTT cancel operation from the user for canceling the PTT mode; and a sign-out unit operable to transmit a release request to the radio base station when the PTT cancel reception unit receives the PTT cancel operation, the release request being for releasing the IP address allocated to the mobile station, wherein when the PTT cancel reception unit receives the PTT cancel operation, the monitor cycle switching unit switches the monitor cycle to a predetermined long cycle which is longer than a cycle at a time of receiving the PTT cancel operation.