[EDIT 20120-05-13: You can follow these steps (though by now they're already fairly outdated), and end up with a mostly functional home phone system connected to a free Google Voice number.  It will break often, as Google makes changes, and your Asterisk box needs updating, and who knows what else.  OR, you can skip all that, spend \$45 and be done with it.  That's what I did.  This little box (OBi110 VoIP service bridge) is one of the best purchases I've ever made.  HIGHLY recommend it.  One trips abroad (Europe and Manila) I used free hotel wifi to make calls over my iPhone with the free OBiTALK client, and as long as the connection was moderately decent, the quality was excellent!]

In 2009 Google purchased a company offering VoIP phone numbers called GrandCentral. That allowed Google to start giving away US phone numbers. The idea was that you would get this new phone number and you could keep it forever - and any calls to that phone number would forward to the phone(s) of your choosing. It also offers voice mail service, and when you get a message, you can have a transcript of that message sent to you via email (as well as the message itself, as an audio attachment.) One of the most powerful features was that you could set up several phone numbers to forward the call to (your cell phone, home phone and office phone). Any incoming call to your Google Voice number would ring on ALL phones at the same time, and the call would be routed to whichever phone you picked up first!

It's a great idea, but it is kind of a hassle to use it for making outbound calls from a regular phone- and if you're not making calls from that number, it's much less likely people will remember to use that one when trying to reach you. You CAN make phone calls from your computer (right in your web browser when connected to gmail), but not too many people like to do that - and it's certainly not convenient for visitors or others in the house who need to use the phone. There was another company (Gizmo5) that allowed you to connect to your GrandCentral number via "voice over IP", so using the two systems was a good alternative to VoIP services like Vonage. Unfortunately Google bought Gizmo5 and suspended new signups while they worked on integrating their technology.

There were some workarounds that let you use the free open-source phone switching system Asterisk to connect to your GV number from a VoIP phone, but they still required phone service through another provider, so it was going to cost you a subscription one way or another.

Lucky for us, the latest version of Asterisk (v1.8) included a module to connect directly to your GV number, giving you a free phone number, potentially for life!

There is already an "everything included" installer available from NerdVittles, but it has WAY more than you need if all you want is to connect your GV number to a regular phone at home. To that end, I'll outline the steps I followed to get a regular wireless phone connected to my Google Voice number for making and receiving calls.
[UPDATE: The NerdVittles package is pretty much awesome.  It's got a lot included, but it's all good.  The install is fast and painless, and a little easier than what I outlined...  I switched to this one at home, and suggest you do the same if you want to tie your Google Voice account to a phone.]

You'll need a spare computer (but it does NOT need to be powerful or have a ton of RAM or disk space - you might even be able to use a free Amazon EC2 instance). You'll also need a SIP/POTS adapter, like this \$30 unit from Grandstream. You'll also need to be nerdy enough to have installed Linux once or twice before, though you do NOT need to be an expert to do this... What follows are all the steps you'll need to set up and configure an absolutely minimal Asterisk installation.

Install CentOS 5.5 - choose the "Server" install. After installation, run the following commands:

yum -y updateyum -y install bison bison-devel ncurses ncurses-devel \zlib zlib-devel openssl openssl-devel gnutls-devel \subversion automake gcc gcc-c++ libtool gnutls \gnutls-devel texinfo kernel-devel

Download Iksemel from http://code.google.com/p/iksemel/, expand in /usr/src, then run:

./configure --prefix=/usrmakemake check [be sure all tests pass!]make install

Download the newest version of Asterisk, expand in /usr/src, then run:

./configure --prefix=/usrmakemake install

Add the following files in /etc/asterisk:

[logfiles]console = warning,notice,error




[general]context=google-in ; Context to dump call intoallowguest=yes[guest]disallow=allallow=ulaw[myaccount-gtalk]username=[YOUR GMAIL USER]@gmail.comdisallow=allallow=ulawcontext=google-inconnection=gtalkconnection


[gtalkconnection] type=clientserverhost=talk.google.comusername=[YOUR GMAIL USER]@gmail.com/asterisksecret=[YOU GMAIL PASSWORD]statusmessage="I am asterisk"status=xaway




[default]exten => s,1,Set(CALLERID(name)=${DB(cidname/${CALLERID(num)})})exten => s,n,Dial(SIP/101,10)exten => s,n, Hangupexten => 101, 1, Dial(SIP/101, 10)[google-in]exten => [YOUR GMAIL USER]@gmail.com, 1, Dial(SIP/101, 180, D(:1))[stations]exten => 101,1,Macro(phone); * Route the call using the google voice bridge[gv-outbound];append an area code if necessaryexten => _NXXXXXX,1,Set(CALLERID(dnid)=1323${CALLERID(dnid)})exten => _NXXXXXX,n,Goto(1323${EXTEN},1);append a 1 if necessaryexten => _NXXNXXXXXX,1,Set(CALLERID(dnid)=1${CALLERID(dnid)})exten => _NXXNXXXXXX,n,Goto(1${EXTEN},1);do our real dialingexten => _1NXXNXXXXXX,1,Dial(Gtalk/gtalkconnection/${EXTEN}@voice.google.com)exten => _+1NXXNXXXXXX,1,Dial(Gtalk/gtalkconnectionEXTEN}@voice.google.com)[users]include => stationsinclude => gv-outbound

You can now start up asterisk (in a sort of test mode) with "/usr/sbin/asterisk -vvvgc". Keep that console connection open until you've confirmed everything works as planned. (When it's all good, you'll find appropriate init scrips in the contrib directory of the asterisk source.) Check to see if you're connected to your Google Voice account with "jabber show connected" from the asterisk console. If you're not connected, then something went wrong...

The only other thing you have to do at this point is configure your SIP client (whether it's a soft-phone or an adapter like the Grandstream device I linked to above.) Then try dialing out, and call your GV number from another phone to test the in-bound calling!

You can do a LOT more with Asterisk, but if you just want to get your GV number linked to a phone at home, this is all you need to do!

Thanks to these folks for their information, what they had to share helped me sort this all out:

  • http://supermario-world.blogspot.com/2010/11/asterisk-18-and-native-google-voice.html
  • http://www.voip-info.org/wiki/view/Asterisk+Speaks+with+Google+Talk
  • http://www.asteriskguru.com/tutorials/asterisk_installation_compilation_centos.html
  • http://jeremy-mcnamara.com/asterisk/how-to-configure-asterisk/
  • http://blog.polybeacon.com/2010/10/17/asterisk-1-8-and-google-voice/