How to get a phone notification if my VPS goes offline?

https://slrpnk.net/post/35723833

How to get a phone notification if my VPS goes offline? - SLRPNK

What are my options for getting alerts on my android phone if services on my VPS becoming unresponsive? My first thought is a simple app that periodically pings domains and gives a notification if any fail. Is there an open source app for that? Or something else?

Uptime Kuma has a number of notification options.

I just had to set one up because the Oracle free tier is hot garbage and needs resetting every day or two.

This is what I use, though I run it on a local server that checks services that it runs (update breaks something, etc) and then also reaches out to the vps (and more) for status checks. Have it check services that are accessible by an access control rule, so the server is protected but you have updates on exactly what is having issues.
I used uptime Kuma with signal-cli for notifications - it’s been rock solid. I also have anotger uptime kuma that uses email as notification option to monitor my primary uptime kuma and the signal-cli.
+1 for Uptime Kuma, I use it with ntfy. If OP doesn’t want a self hosted solution, there’s UptimeRobot - essentially SAAS Uptime Kuma, but with a free tier.
Same here. Run it on my VPS to test my outside reachability and inside to check against my VPS.
I haven’t touched my Oracle in months. What exactly is happening?

No idea. The dashboard says it’s running and fine, sometimes the shell console is ok, sometimes it’s also bogged down, but the instance is completely non-responsive from outside. The other instance I have in the same tenancy but a different fault domain is perfectly fine, so I don’t know if that’s related somehow. I’m running Pangolin on it to forward a couple things, one of them is higher throughput than I had expected, but rebooting makes it come back, so I don’t think they’re throttling me.

I’m kind of at a loss, but I threw together a webhook to use their API to reboot it when it stops responding and that seems to be working while not actually addressing whatever the root cause is.