voip? where to start?

i have been using textnow for some time now and i have been having some issues using this service for my needs.

i thought maybe i can make a voip service my own, much like building a smtp server.

i found this subreddit, and it seems this is a thing, and i want to do this as well, but dont know where to start.

it would be appreciated if someone can show me a little place to start. maybe some documentation on voips?