A server stores a table of correspondence between phone numbers and dynamic addresses of an internet protocol assigned or to be assigned to cellular phones. When the server is notified of a phone number of a receiving cellular phone by a transmitting cellular phone, it searches the table to obtain the dynamic address corresponding to the phone number of the receiving cellular phone. The server then notifies the transmitting cellular phone of the obtained dynamic address. The transmitting cellular phone can thus identify the dynamic address assigned or to be assigned to the receiving cellular phone. Thus, the transmitting cellular phone is enables to communicate with the receiving cellular phone through the internet protocol.