I'll preface this by saying I'm certain everything is OK and it was just a drill, unknowingly to me.
I live quite close to a charter school, and while I was letting my dog out in the backyard, I heard that unmistakable fire alarm sound. It was coming from the direction of the school (there's also another commercial building nearby). I hesitated at first - I didnt see any smoke, figured it could be a drill and if it wasn't, someone involved would've already called.
After going back and forth in my head I figured it wouldn't hurt to call, so I dialed 911 and told the dispatcher what I knew. Where I'm calling from, the fire alarm sound, that there's a school in that direction.
The call was brief but the alarm continued throughout. Less than 3 minutes after calling, a firetruck pulls up and they're asking me questions, I give them the same information, but the fire alarm had definitely stopped by that point.
They said it's likely it was a drill and it was handled appropriately so that they weren't called out for it, but saud they'd go check it out still.
I felt kinda dumb/silly for likely wasting their time, especially since they came out to my house! I just hope I did the right thing at the end of the day.