So, you have several options here, depending on if you're "just" looking for VOIP or if you're also looking for Video Chat. I've listed a couple, but there are likely to be MANY more options.
For video chat, I've used Jitsi, which, when checked today (2021-03-02) has a self hosting guide on their Github repository. https://jitsi.github.io/handbook/...
GuerrillaMail.com is very similar to the sites you mention, and the core of it appears to be this open source project:
It's not a turnkey solution, but it could be a good start toward building your own version.