An object of the invention is to provide a communication system that enables telephone communications to be quickly performed by a simple operation. A user activates a telephone directory application program on a PDA terminal, and the PDA terminal transmits a destination telephone number to a gateway together with a request for initiation of a call to that destination telephone number and an IP address of a NET telephone terminal that the user desires to use. When an off-hook condition of the NET telephone terminal is detected, the gateway initiates a call to the previously received telephone number and establishes a communication link to a destination NET telephone terminal accommodated in a digital public telephone network. The communication link can be established in the same manner even if the destination telephone terminal is an ordinary telephone terminal accommodated in a public telephone network.