A somewhat similar system used in modern radio systems (well, radio systems that are now outdated, but still in widespread use) is two-tone sequential paging or the related QuikCall. These are in-band audio tones used to cause some receivers to open squelch as a form of selective calling.
The method is predominantly used in public safety radio systems, where tone paging may be used to cause a fixed receiver in a station to open squelch (triggering an alarm) or cause a portable paging receiver (like a Motorola Minitor) to open squelch as a simple form of paging. Rural VFDs often rely on the latter to call out volunteers.
Anyway, the world of in-band signaling in voice communications is really pretty interesting, and while digital systems have reduced the need for in-band there are still a lot of real-world systems that rely on it for compatibility with legacy equipment. The telephone system has largely moved on (except for various niche uses of DTMF) but in-band signaling is still pretty common in radio systems, where upgrading to a digital system is high cost and comes with its own limitations.
Another example off-hand: most large-area public address systems, the type of thing the military often calls Giant Voice and might be used for tornado warning in some areas, use either DTMF or SelCall (five-tone) on the control radio channel (which in many cases is the same radio system used by public safety portable radios) to cause the loudspeakers to activate. A particular oddity of a lot of these systems is that they have different generations of receivers that use different methods, so sometimes you'll end up hearing the tones played aloud if a receiver opens squelch in reaction to something earlier in the sequence.
And a last example, also usually seen in public safety - some radio schemes like MDC transmit a burst that sounds like a roger beep whenever the PTT is released, but is actually a short data packet that identifies the transmitting radio. This allows a specially equipped base station to show e.g. a dispatcher who was just transmitting. Sometimes the same signaling system is used to implement other features like 'man down' detection where a radio signals if it is not upright or has not moved for a period of time (common safety feature in fire departments), but sometimes these kinds of systems also just use a sample-based speech synthesizer if there's no digital system already in use.
Oh, I can't resist one more. The "buzz," repeated three times, which proceeds emergency alert messages in the US is actually a short data packet which gives certain metadata about the alert and causes hardware in receiving radio stations (the system operates a bit like a "calling tree") to cut out the program audio and switch over the received alert. The two-tone beep which follows is a legacy system for the same purpose, although today no such receivers should still be in use. It's amusing that many phones use that two-tone signal as their ringtone for emergency alerts considering how divorced that is from its original purpose, but it's quite recognizable. Emergency alert messages should be followed by a three shorter data bursts which inform receivers that the alert is over. A closely related scheme is used by NOAA All-Hazards Radio to cause dedicated "weather emergency" receivers in people's homes to open squelch, these are more common in tornado-prone areas but generally work throughout the US. As the name suggests, All-Hazards also retransmits alerts from agencies other than NOAA including emergency alert system messages.
The NOAA system has a name: SAME, or Specific Area Message Encoding[1]. As the name implies, those three bursts actually contain a whole packet of information regarding the incoming message, including areas of effect, hazard type, duration, source of the alert, etc. There are pieces of software out there that let you decode the packets, and some let you encode your own, like minimodem[2]. Do be aware that transmitting valid SAME packets over broadcast media tends to be very poorly viewed by the FCC because it actually triggers mechanisms in the broadcast system, as GP noted.
> A particular oddity of a lot of these systems is that they have different generations of receivers that use different methods, so sometimes you'll end up hearing the tones played aloud if a receiver opens squelch in reaction to something earlier in the sequence.
Good grief, I'm getting old...
First thing I thought of after reading that:
The method is predominantly used in public safety radio systems, where tone paging may be used to cause a fixed receiver in a station to open squelch (triggering an alarm) or cause a portable paging receiver (like a Motorola Minitor) to open squelch as a simple form of paging. Rural VFDs often rely on the latter to call out volunteers.
Anyway, the world of in-band signaling in voice communications is really pretty interesting, and while digital systems have reduced the need for in-band there are still a lot of real-world systems that rely on it for compatibility with legacy equipment. The telephone system has largely moved on (except for various niche uses of DTMF) but in-band signaling is still pretty common in radio systems, where upgrading to a digital system is high cost and comes with its own limitations.
Another example off-hand: most large-area public address systems, the type of thing the military often calls Giant Voice and might be used for tornado warning in some areas, use either DTMF or SelCall (five-tone) on the control radio channel (which in many cases is the same radio system used by public safety portable radios) to cause the loudspeakers to activate. A particular oddity of a lot of these systems is that they have different generations of receivers that use different methods, so sometimes you'll end up hearing the tones played aloud if a receiver opens squelch in reaction to something earlier in the sequence.
And a last example, also usually seen in public safety - some radio schemes like MDC transmit a burst that sounds like a roger beep whenever the PTT is released, but is actually a short data packet that identifies the transmitting radio. This allows a specially equipped base station to show e.g. a dispatcher who was just transmitting. Sometimes the same signaling system is used to implement other features like 'man down' detection where a radio signals if it is not upright or has not moved for a period of time (common safety feature in fire departments), but sometimes these kinds of systems also just use a sample-based speech synthesizer if there's no digital system already in use.
Oh, I can't resist one more. The "buzz," repeated three times, which proceeds emergency alert messages in the US is actually a short data packet which gives certain metadata about the alert and causes hardware in receiving radio stations (the system operates a bit like a "calling tree") to cut out the program audio and switch over the received alert. The two-tone beep which follows is a legacy system for the same purpose, although today no such receivers should still be in use. It's amusing that many phones use that two-tone signal as their ringtone for emergency alerts considering how divorced that is from its original purpose, but it's quite recognizable. Emergency alert messages should be followed by a three shorter data bursts which inform receivers that the alert is over. A closely related scheme is used by NOAA All-Hazards Radio to cause dedicated "weather emergency" receivers in people's homes to open squelch, these are more common in tornado-prone areas but generally work throughout the US. As the name suggests, All-Hazards also retransmits alerts from agencies other than NOAA including emergency alert system messages.