Setting up VoIP at home using Asterisk instead of Vonage, etc

I'm a fairly seasoned computing professional with intermediate to advanced Linux and mainstream networking skills. But I know little to nothing about VoIP other than knowing that it's... well, Voice over IP. And little to nothing about the telephone industry at large as far as how it works internally, etc.

So I'm thinking about getting VoIP in the home via Vonage or something like that, and I'm thinking, why can't I just build my own VoIP system using Asterisk that the rest of the world can see? It seems you'd need a way to register your telephone number and a way to route calls to other numbers -- this is what the "phone company" does-did. So why can't just anyone do it? What I'm asking is, is there any way to "register" a phone number and a VoIP installation at home the same way one goes about registering a domain and running a Web server at home on a machine I have hands-on control of?

Can anyone enlighten me on how this works? I realize I'm probably asking in not EXACTLY the right forums but (1) I don't even know what forums would handle VoIP in a situation like this ("hobbyist" vs. SOHO), and (2) with the core of my question pertaining to Asterisk, which uses Linux, I figure someone out there in Linux land can point me in the right direction. Why should I hand Vonage $15-month or more if I can just install something myself, register it somewhere, and handle calls myself, just a like a Web server or any other internet service?

Thanks! -ceo