Could you tell me the best practices on the error notification algorithm?

Question

Notifications

There is a website functional testing service. It answers the question "is the site broken" for hundreds of domains. You need to attach notifications to it via SMS and calls. I can't think of an algorithm by which to send these notifications.
The requirements are approximately the following:
- notify in time;
- reach out to a person, even if he is stoned dancing at a Prodigy concert;
- save money for sending sms and calls;
- do not spam a person when he already knows about the problem;
- don't miss anything important.
There are so many situations when something can go wrong that I do not undertake to come up with a reliable solution. I would like to use someone else's algorithm, tested in practice.

Answer 1

- notify in time;

On the fact of the appearance, send sms.
Return to reality, if the attendant is at the concert - he must warn about this so that someone replaces him, otherwise - nothing.
Zabbix is ​​good at acknowledging problems. If the duty officer raised the problem, you do not need to send sms after that.
More metrics are needed.