That I know of, I don’t claim to know everything.
It’s been discussed elsewhere here, push notifications go though Google/apple servers, and for billing/security/spam reduction the sender must be trusted by them. Even if I build an opensource solution that supports push I won’t automatically have the credentials to push notifications to it. That’s just one part (from apple/google to device), the other part has to do with freeswitch/fusion sending the signal to apple/google so they send to the phone, there are some recipes floating around, I think even on this forum, but they too are guides and require you to be setup with firebase (middleman service, may have the name wrong) or apple/google as a dev.
Ringotel/Acrobits use their own infra as proxies such that we need not modify our servers, and the pushes are sent from their developer accounts to clients signed by them.
Do your research, learning is fun. Also put effort into understanding things conceptually, many questions answer themselves when you think things through.