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.
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.
It's doable but it would be custom firmware and it's not really necessary. Two way paging isn't really worth doing because then you need a massive device with a massive battery, or something that uses uncontrolled mobile phone networks (and generally still has a massive battery, that lasts about a day).
You wouldn't even need particularly good encryption, you'd just need something adequate to stop casual eavesdropping really - "keep them busy for half an hour" would stop people from sniffing the POCSAG traffic and tweeting it, so that people show up at incidents and hang around filming it on their phones.
This incidentally is what a guy in England got arrested for a few years ago, exactly that. It's perfectly legal to listen to and decode pager messages (or any other radio messages), you're just not allowed to pass them on to people or act upon them, and posting them on twitter and then going round to rubberneck at the ongoing incident very much ticks those boxes. As with so many things in the UK, to paraphrase Aleister Crowley, "Don't Be A Dick shall be the whole of the law".