I'm working in a project for my local fire station to receive GPS data via VHF radio (widely used by emergency services in my country). We are trying with DTMF, this seems interesting.
Newer digital modes do much better than this older AFSK system, but FSK does work pretty well. Some newer things to look into:
https://en.wikipedia.org/wiki/WSPR_(amateur_radio_software)
FT8: https://en.wikipedia.org/wiki/WSJT_(amateur_radio_software)
The more interesting modes (WSPR, FT8) are designed for high-loss channels, so have a lot of error correction. For example, I can use 5W of power, indoors, to communicate across the country with it. For high-bandwidth VHF channels, it doesn't matter what you use really. The goal would be to find something that already exists and is easy to integrate, and there are a lot of options.
All you need is a dtmf decoder and a small transmitter to set the off those large sirens on high poles throughout that sprinkle the country in small towns and probably large cities