A network system comprising: a mobile phone network having a first base station control device as a source base station control device; a second base station control device as a destination base station control device; and at least one switch (MSC1, MSC2) for connecting the first base station control device and the second base station control device (RNS1, RNS2) characterized in that when a mobile phone terminal (UE-a) as a calling terminal makes a calling request and the first base station control device receives the calling request from a source base station (CABS1) connected thereto through an IP network, the first base station control device adds, to the calling request, identification information about the Internet Protocol, IP, network to which the source base station (CABS1) is connected and an IP address of the source base station, when the second base station control device receives the calling request through the at least one switch (MSC1, MSC2) and sends the calling request to a mobile phone terminal (UE-b) as a called terminal through a destination base station (CABS2) connected to the second base station control device through an IP network, the second base station control device checks whether the identification information about the IP network that is contained in the calling request coincides with identification information about the IP network to which the destination base station (CABS2) is connected, and when the two pieces of IP network identification information coincide with each other, the second base station control device informs the destination base station (CABS2) of the IP address of the source base station contained in the calling request and also informs the source base station of an IP address of the destination base station, and the first and second base station control devices (RNS1, RNS2) provide an instruction to the source base station (CABS1) and the destination base station (CABS2) respectively to establish a data transmission path between the calling terminal (UE-a) and the called terminal (UE-b) which directly connects the source base station (CABS1) and the destination base station (CABS2) through the same IP network instead of via the mobile phone network. a mobile phone network having a first base station control device as a source base station control device; a second base station control device as a destination base station control device; and at least one switch (MSC1, MSC2) for connecting the first base station control device and the second base station control device (RNS1, RNS2) characterized in that when a mobile phone terminal (UE-a) as a calling terminal makes a calling request and the first base station control device receives the calling request from a source base station (CABS1) connected thereto through an IP network, the first base station control device adds, to the calling request, identification information about the Internet Protocol, IP, network to which the source base station (CABS1) is connected and an IP address of the source base station, when the second base station control device receives the calling request through the at least one switch (MSC1, MSC2) and sends the calling request to a mobile phone terminal (UE-b) as a called terminal through a destination base station (CABS2) connected to the second base station control device through an IP network, the second base station control device checks whether the identification information about the IP network that is contained in the calling request coincides with identification information about the IP network to which the destination base station (CABS2) is connected, and when the two pieces of IP network identification information coincide with each other, the second base station control device informs the destination base station (CABS2) of the IP address of the source base station contained in the calling request and also informs the source base station of an IP address of the destination base station, and the first and second base station control devices (RNS1, RNS2) provide an instruction to the source base station (CABS1) and the destination base station (CABS2) respectively to establish a data transmission path between the calling terminal (UE-a) and the called terminal (UE-b) which directly connects the source base station (CABS1) and the destination base station (CABS2) through the same IP network instead of via the mobile phone network.