Many depts still use the old two-tone sequential method (as heard on E!) of activating station alarms (aka:ringdown). At "Station 51", I believe the horn that activates after the station tones are received is a Federal "Vibratone" buzzer. I worked at a dept that had one at every station, and boy are they loud. The telephone tones, (aka: DTMF) type of signaling over-the-air has gained in popularity, as there are more tone combinations, often in groups of three or four tones. DTMF also can be transmitted and received faster, thus minimzing the time of the tone-out procedure. Some depts transmit their station alarms via phone line, so their initial dispatches are rarely heard on the radio.