An intercom system comprising an external station, which comprises in turn calling means and means for generating a ring signal, apt for producing and sending an electric signal, in response to operation of said calling means, to a plurality of internal intercoms apt to receive said electric signal and send it to means for acoustical amplification and reproduction contained in said internal intercoms. According to the invention, the electric signal generated by the ring signal generating means (DDD) is a signal with a predetermined duration and that selecting means (C, RL1) are provided, which are apt to establish connection of the means for acoustical amplification and reproduction (SP1) of the internal intercom (CI) selected through the calling means (KEY) to the ring bell signal generating means (DDD) and maintain connection for the whole predetermined duration of the electric signal generated in the external station (PE).