Here is a rudimentary alerting system I could write quickly in shell and execute on my laptop:

  while true; do echo ok | nc -l 8000; for i in 1 2 3 4; do printf '\a'; sleep 1; done; done

While the above command worked fine on macOS with the default /usr/bin/nc, on Debian 11.2, I had to modify the above command as follows to make it work:

  while true; do echo ok | nc -q 1 -l -p 8000; for i in 1 2 3 4; do printf '\a'; sleep 1; done; done

Now anytime someone connects to port 8000 of my system by any means, I will hear 4 beeps! The other party can use whatever client they have to connect to port 8000 of my system, e.g., a web browser, nc HOST 8000, curl HOST:8000, or even, ssh HOST -p 8000, irssi -c HOST -p 8000, etc.

If your port 8000 is already occupied, I recommend port 41327 as an alternative for this alerting service. After all, 41327 reads "ALERT" in leetspeak.

By the way, visit http://susam.net:8000/ right now to send me some beeps! :)