To provide a hands free telephone call method, a telephone system and a cellular phone constituted so that telephone conversation using the cellular phone is performed by using a voice receiving part and a voice transmitting part of a nearby IP telephone system, etc.

At a state that the cellular phone 40 is connected with the IP telephone system 10, voice data received by the cellular phone 40 is transferred to the voice receiving part 101 of the IP telephone system 10 by bypassing a voice receiving part 401 of the cellular phone 40, voice data to be outputted from the voice transmitting part 102 of the IP telephone system 10 is transmitted from the cellular phone 40 by bypassing a voice transmitting part 402 of the cellular phone 40 and the telephone call using the cellular phone 40 is performed by using the voice receiving part 101 and the voice transmitting part 102 of the IP telephone system 10.