The proper name for the feature is "Channel Marker".
SDSO had this feature decades ago on the UHF 453MHz system. It was a DTMF-based device that produced a rapid series of DTMF "0" tones to indicate there was emergency traffic in progress.
Unfortunately SDSO lost that capability when things migrated to the RCS, as the channel marker was not a supported feature in SmartZone 3.0. When the system was upgraded to 4.1 and beyond, they *could* have brought it back, but apparently they elected to bring it back with the move to MCC 7500.
San Diego Police had the same thing on their SmartNET system, the difference being they used DTMF "9". They were able to have this on a trunked system as a "Special Product", it was not standard feature and was custom-built for them. When they migrated to SmartZone 4.1, they lost the DTMF channel marker and began using the channel marker integrated into Centracom Gold Elite (since upgraded to MCC 7500), which is the single tone.
There is everything you never wanted to know about channel markers in San Diego County.