Infrastructure Stability Report
Its very common to provide a StatusPage (like Statusy.co or StatusPage.io). There are numerous examples of major providers having them:


https://status.github.com/messages
https://status.bitbucket.org/
https://status.aws.amazon.com/
https://azure.microsoft.com/en-us/status/
https://api.twitterstat.us/


You could provide the same status page to your customers. When there is a service disruption, you could manually update the status while providing detailed, customer-facing information. Or you could hook it into your Zabbix monitoring to have the status auto-updated.

This also lets your customers see a history of incidents and average uptime of your service.

There are also self-hosted, open-source solutions that you could implement:


https://cachethq.io/
http://www.stashboard.org/


Although, you'll really want to host those outside your existing infrastructure so that your StatusPage can always be reached, even if you're down. If you go the self-hosted route, I recommend hosting the status page at a different provider: Digital Ocean, AWS, etc...