running voip on third party server wtthout third party voip service

[asinshesq]

I currently have a number of yealink voip phones in my house that tie into a 3cx server I am running on a local laptop. The phone service to the outside world is analog (Fairpoint) and I use a patton fxo gateway to bridge from my 3cx system to the outside.

I realize that this is a newbie type question but I am curious: is it possible to sign up for a third party voip server (to replace my local 3cx server) without actually signing up for a third party voip service? Would that be a crazy thing to do (as opposed to actually signing up for a real voip service)?

If I were to go with a true third party voip service rather than just a server, would I still be using a gateway or would I connect the voip phones directly to the third party service?