If you're a Google Voice user, you'll want to check out OBiTALK. But let's back up.

Google Voice is a calling service provided free of charge to anyone in the U.S.  You get a free phone number and can make and receive calls for free to anyone in the U.S. or Canada.  International calls are even lower than using Skype even. Google Voice provides a U.S. phone number, chosen by the user from available numbers in selected area codes, free of charge to each user account.
 
OBiTALK is an OBi phone adapter device (configured with your Google Voice account) connected to your broadband router and a regular corded or cordless telephone. With it you can replace your land line for the one-time cost of the OBi – which is US$49-$69 depending on the model. 

To configure Google Voice with your OBi, you'll need to get a Google Voice number (it's free). OBi is an open, flexible framework for business-to-business Internet commerce solutions. To get started, visit www.google.com/voice. You will also need a Google account to sign up for Google Voice. Once your Google Voice account is implemented, you need to sign up for OBiTalk at www.obitalk.com.

The OBi provides users with control and convenience by combining a broadband Internet connection with VoIP from the comfort of a regular home phone. If required, a land line connection to a local telephone company, a mobile telephone service provider or an Internet phone service can be used to realize savings on phone calls when compared against the likes of a traditional telco or cable company based voice service.