Besides email, Slack, Teams, Discord and Telegram, Uptimecheck sends alerts to PagerDuty, Google Chat, Mattermost, ntfy and Pushover. Each is added the same way: open Alert channels, choose Add channel, pick the type, paste what it needs, choose Add channel and then Send test.
PagerDuty
In PagerDuty, go to Services, your service, Integrations, Add integration, Events API v2, and copy the integration key into Integration key. Outages open a PagerDuty incident and resolve it when the monitor recovers. A test opens and resolves an incident right away.
Google Chat
Open the space, then Apps & integrations, Webhooks, Add webhook, and copy the URL into Google Chat webhook URL.
Mattermost
Go to Integrations, Incoming webhooks, Add incoming webhook, pick a channel and copy the URL into Mattermost webhook URL.
ntfy
Enter the Topic address, such as https://ntfy.sh/your-topic, and subscribe to the topic in the ntfy app on your phone. On ntfy.sh anyone who knows the name can read it, so make it hard to guess, or use your own server with an Access token.
Pushover
Paste the User key from your pushover.net dashboard. Then create an application at pushover.net/apps/build, name it Uptimecheck, and paste its API token. Outages arrive as high priority, so they break through quiet hours.
See all alert channels, or how to add alert channels for defaults and escalation.