A somewhat similar system used in modern radio systems (well, radio systems that are now outdated, but still in widespread use) is two-tone sequential paging or the related QuikCall.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.