You will need a Google voice account to do this. You can get one for free by having a normal Google account, and visiting the Google Voice page.
The top most portion of this page: http://pbxinaflash.com/forum/showthread.php?t=10825 will help you setup the Google Voice portion on the Google side. Once you see the screen shots on that page, you can then use this configuration, or read up if you have their software to continue.
Just my working configuration files for using a google voice number on your asterisk server.
gtalk.conf
[general] context=googlein bindaddr=0.0.0.0 externip=xxx.xxx.xxx.xxx ;stunaddr=mystunserver.com allowguest=yes [guest] disallow=all allow=ulaw connection=asterisk context=googlein
jabber.conf
[general] debug=no autoprune=no autoregister=yes ;collection_nodes=yes ;pubsub_autocreate=yes ;auth_policy=accept ;---------------------------------------------- [trunkname] type=client serverhost=talk.google.com ;pubsub_node=pubsub.astjab.org username=[email protected]/Talk secret=secretpassword priority=1 port=5222 usetls=yes usesasl=yes ;buddy=[email protected] ;distribute_events=yes status=Available statusmessage="I am an Asterisk Server" timeout=100 keepalive=yes
Add following block to extensions_custom.conf
[googlein]
exten => _[0-9a-z].,1,Noop(Incoming Google Voice call for ${EXTEN})
exten => _[0-9a-z].,n,Set(CALLERID(name)=${CUT(CALLERID(name),@,1)})
exten => _[0-9a-z].,n,GotoIf($["${CALLERID(name):0:2}" != "+1"]?notrim)
exten => _[0-9a-z].,n,Set(CALLERID(name)=${CALLERID(name):2})
exten => _[0-9a-z].,n(notrim),Set(CALLERID(number)=${CALLERID(name)})
exten => _[0-9a-z].,n,Wait(1)
exten => _[0-9a-z].,n,Answer
exten => _[0-9a-z].,n,Wait(1)
exten => _[0-9a-z].,n,SendDTMF(1)
exten => _[0-9a-z].,n,Goto(from-trunk,gv-incoming-${CUT(EXTEN,@,1)},1)
exten => h,1,Macro(hangupcall,)
[gvoice-trunkname]
exten => _X.,1,Dial(Gtalk/username/+${EXTEN}@voice.google.com)
exten => _X.,n,Noop(GVoice Call to ${EXTEN} failed)
exten => h,1,Macro(hangupcall,)
Custom Trunk Configuration:
Trunk Name: GVoice-trunkname
Outbound Callerid: “Name”
Custom Dial String: local/$OUTNUM$@gvoice-trunkname
Outbound Route Configuration:
Use the GVoice trunk.
Incoming Routes DID will be:
gv-incoming-username (without @gmail.com)