When receiving an extension connection request from a first mobile terminal, a first route selection device selects a first extension connection route from among a plurality of extension connection routes and transmits the type of the first extension connection route and calling-side address information used on the first extension connection route to a second route selection device. The second route selection device selects a second extension connection route from among the plurality of extension connection routes and transmits the type of the second extension connection route and called-side address information used on the second extension connection route to the first route selection device. If the second extension connection route coincides with the first extension connection route, an extension connection is established between the first mobile terminal and the second mobile terminal via the first extension connection route.