Our community has old air-raid sirens that can be activated to warn the public of an actual or pending emergency.
Of course, NWS/NOAA weatherband radios usually have an alarm/alert function. And the newer ones can be set with a code that allows it to ignore alerts for other regions. So, from a technical perspective, it's already a pretty good alert system (ours gets me to sit up straight and take notice even while asleep in bed).
The problem is how few people own these inexpensive radios, or are even aware of them. Sailors and surfers seem to be dialed in, but not a very large portion of the general public. By now you'd think that Weatherband radio would be standard in vehicles, but very few manufacturers include them (Saab and Subaru are two exceptions). Instead they push the fancy satellite-radios that are inoperable without a paid subscription.
We in the U.S are already paying for one alert system, so I hope the alternative cell-phone approach doesn't cost us much. I'd rather see the money put into making the general public more aware of the existing system..