In a network system including base stations accommodated in a mobile phone network through an IP network, when a mobile phone terminal as a calling terminal makes a calling request and the calling request from the calling terminal is sent through a source base station to the mobile phone network and then from the mobile phone network through a destination base station to a mobile phone terminal as a called terminal, it is judged whether the source base station and the destination base station are both connected to the mobile phone network through the same IP network, and when the source base station and the destination base station are both connected to the mobile phone network through the same IP network, a data transmission path is established between the calling terminal and the called terminal that directly connects the source base station and the destination base station through the same IP network and without via the mobile phone network.