*** Update ***
Unfortunately, Google has decided to end support for the XMPP protocol in May of 2014. I will keep the post here in the hope that Google releases a new API, but also to share that information with any searchers. As of April 2016, the service is still operational, but there’s no telling for how long that will be the case.
Earlier this year, the company I work for, Laird Technologies, converted their phone system to Cisco IP phones. In the process they threw out all of their old IP phones which were used with their Asterisk system. I acquired these, all Grandstream products (mostly GXP2020’s, 1 or 2 GXP2000’s and even a HandyTone-286!) and I had some ideas about what I could do with these, but I mostly just wanted to play around: set up phones around the house, make a doorbell/intercom system, etc. After I started reading and Asterisk and how people are using it, I got the idea to set up my own “land-line” service using Google Voice.