What is the best way to receive notifications from a downed server on a cell phone?

Question

System administration

There is a web project with self-diagnostics. I would like to quickly receive a notification from the server to the cell phone if this self-diagnosis is triggered. You can pay for the service of sending SMS, but I suspect there are ready-made and free solutions in this area. What do you advise?
EDIT: We are not talking about the network falling off, but about situations when a worthless exception is caught and you need to urgently tell the admin about it.

Answer 1

Make a bot in Telegram and send messages via API, for example.

Answer 2

You buy the cheapest VDS for 130 rubles (it can be cheaper if you look), write a script for a couple of lines that will send requests to your server at regular intervals.
If you have hosting with some kind of site, it is quite possible to do this on it.
In case of no response (the server is down or something else), the script will send you SMS and email.
If you need SMS, you send the minimum payment to some SMS mailing service and send it through the API, you can use sms.ru/free for free and a duplicate for yourself.
You can find ready-made free solutions, but they do not work very efficiently and alerts often come in a day, and only by mail.
If you really need only alerts from self-diagnosis - then the same sms.ru/free.
But you won't get them if the server actually crashes, along with self-diagnostics.