AS-B (810) measures a telephone call time from a time point when a telephone call, which is audio-originated by UE (100), starts. When a time obtained by subtracting the telephone call time from a time for which the UE (100) can telephone-call becomes a predetermined time, the AS-B (810) issues a multiplex instruction to FAP (200). When having received the multiplex instruction, the FAP (200) multiplexes a sound, which is based on a sound source file stored in advance, with audio data to be transmitted to the UE (100), and transmits the audio data, with which the sound has been multiplexed, to the UE (100).