How can I test whether an incoming call is from Google voice or a direct call to my phone?

I'm interested in writing a simple Android app to block incoming calls that are not being routed through Google Voice. Is there I way to check 开发者_开发百科this?

Google voice gives the option of caller id through from original caller, or caller id from Google Voice number. The user could use the latter option, and the application would block, or dismiss, any calls not from that number.

Of course the downside to this option is there will be no caller id information for incoming calls.

Plus side is the phone will only ring for Google Voice calls.