Here is another option (thank me later):
If you want incoming and outgoing calls for free with 3CX....here's the dirt (411)...
First signup for Google Voice if not already, make sure Google Voice is configured to forward to Google Chat in the Settings. I had to add my mobile number and verify it first, GV requires a physical phone, make a test call with mobile first, just uncheck it (in forward settings screen) once you go thru initial calls with GV.
if all good then time to configure SipBri, http://www.sipbri.com, give sipbri the GV credentials. Once you have that configured, jot down the sip server info/credentials, etc...
Wait about 15min then first test with 3CX Phone then configure your 3CX (trust me on this). Must dial 1 first when using the sipbri service, for those in USA, ex.. 1-513-666-6666 in your dial rules.
Call your Google Voice number and test or call someone.
Allows us to fully test without a true VOIP Provider, Spring 2012 we plan to go with 3CX.
Have fun!
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UPDATE: This may change but use sip2.remacservices.net to get full outbound/inbound as the sip.remacservices.net does not allow folks to dial inward to your Google Voice number.