A call system for performing a voice call by an Internet call function using a known telephone terminal. In a multifunction machine serving as the telephone terminal, the output destination and the input source of audio signals input/output from/to a transmitter/receiver may be switched to an audio I/F. In the switched state, audio based on audio signals input/output from/to an outside source through the audio I/F is output/input from the transmitter/receiver. Therefore, when the multifunction machine is connected to a PC having the Internet call function such that audio signals can be input/output through an audio cable, an indirect audio call by the Internet call function with the transmitter/receiver is available. When an incoming call from a telephone line network is received, reception of the incoming call is notified so that a user can confirm the reception of the incoming call during the voice call by the Internet call function.