As a matter of fact, and despite the thread's subject (which I'll change in "Voice Routing Services"), the "voice service" should be the very base for other services too, at the same time as it's own "base service" (e.g. "traditional" phone-to-phone).
Thinking at it, seems more like a "voice routing" (server(s)) which can be used, by means of "plug-ins", by various applications which require to have a voice support.
That way, two or more "terminals" (e.g. smartphones) running a "voice client" can "plug-into" the voice routing servers to contact each other.
These terminals (clients) can also "plug into" (use) other servers, like streaming ones, image ones, to have music, video-conference, etc. A 3D client can plug-into the voice servers to (voice) contact one or more 3D client(s) or even other kind of "terminals" with enough "capabilities" to answer the call/inclusion (e.g. smartphones, PC with mic/headphones and running client/plugin, etc.).
I have to say I'm not an expert of these kind of services in detail: I just saw living applications, as many others, where the "voice support" was not integrated with the client itself but can be used on demand (like a plug-in).
The model may be the usual TCP/IP stack, with address:port where the voice routing servers are daemons into hardware servers, like the majority of other servers/services.
The authentication layer have to be studied, to be centralized (Single Sign On like), meet security needs, and permit the accounting of the services (where "accounting" doesn't necessarily means "pay service", as a fee can be set to "0 coins", but ready to be used on need, e.g. to make external users to pay for our services).
This is just an overview of something which may be useful to have, and I'm sure there is something already done in that way: TeamSpeak, Demonsaw, Skype, etc.
The interesting thing is how to implement our own "voice service", mapped over the internet, to use right "now", when we're still on Earth, but even by our successors, when they'll be into space.
There is plenty of services offering "free calls" as soon as you're not going to use the phone network: as far as I can see, services over internet are usually free of charge.