Patent ID: 7822004

Claim:
A system for establishing a data transmission line between mobile telephone terminals, comprising: a plurality of radio base transceiver stations accommodated in a mobile telephone network through an identical IP (Internet Protocol) network; and a call agent device provided to the IP network, wherein: the call agent device comprises: a storage portion that stores counterpart information including identification information of a mobile telephone terminal on a called side and an IP address of a radio base transceiver station accommodated in the IP network, the radio base transceiver station being available to the mobile telephone terminal on the called side; a receiving portion that receives a calling request issued and transmitted by a mobile telephone terminal as a calling terminal, from one of the plurality of radio base transceiver stations as radio base transceiver stations on a calling side; a retrieving portion that obtains identification information of a mobile telephone terminal as a called terminal, and reads out, from the storage portion, an IP address of a radio base transceiver station corresponding to the identification information obtained, the identification information being included in the calling request; and a transmitting portion that transmits the calling request, in a case where an IP address of a radio base transceiver station is read out by the retrieving portion, to the IP address by assuming that a radio base transceiver station having the IP address is a radio base transceiver station on a called side; and the system establishes a data communication line between the radio base transceiver station on the calling side and the radio base transceiver station on the called side through the call agent device, in a case where the called terminal is subordinated to the radio base transceiver station on the called side itself, the data communication line being established for transmitting data to be exchanged between the calling terminal and the called terminal, the data communication line connecting the radio base transceiver station on the calling side and the radio base transceiver station on the called side to each other through the IP network without routing through the mobile telephone network.