There's a company in England called "Cascode" who make firefighter alerters. These are really basic "beeper" pagers, which you can program to have a bunch of different tones and LED patterns based on the RIC and Subcode.

I look after several thousand of these across several hundred paging sites.

They're relatively inexpensive (70 quid or so in quantity) and they last about six weeks on a commonly-available AA battery. The batteries go flat enough to trigger the "low battery" beep at about 3am, for some reason. I don't know why.

There's no messaging involved, although the encoders are capable of sending a text string. The message is "get up and get down to the fire station right now", which generally needs no further explanation. POCSAG is unencrypted, so there would be privacy concerns with sending actual incident information in the clear with it.

While we're on the subject of old tech, until BT finally cut the last of them off, we use dialup modems to control the encoders (not dialup internet, just a hundreds-of-miles serial cable) as a backup, and dot-matrix printers to print out a hardcopy message for the crews to pick up.

All very low-tech. All very fixable. All stays working if you don't mess with it.

https://cascode.co.uk/products/2ar2-and-2ar3/

Encryption is easily doable even with one way pagers. With one way you will lose the perfect forward secrecy option but that's usually ok.