A network terminal includes an operating system on which a plurality of user environments can run in parallel, a plurality of calling units corresponding to the plurality of user environments respectively, and a plurality of cooperative units corresponding to the plurality of user environments respectively. Priorities different from one another is assigned to the plurality of user environments respectively. Each calling unit is configured to convert a first telephone signal received from an interface and a second telephone signal received from an IP telephone terminal so that an IP telephone communication is performed between the interface device and the IP telephone terminal. Each cooperative unit is configured to communicate the interface with the corresponding calling unit in response to a calling request received from the IP telephone terminal. A cooperative unit corresponding to a user environment to which the highest priority among the plurality of user environments is assigned communicates the interface with the corresponding calling unit.