I have not found a way to get it to work directly with google voice... I have had to get a "middle man" service that has a dial in number that has 2 way communication with Google voice and linphone separately (I recommend IPKall)... unfortunately, Google Voice does not have a way to be directly connected to any SIP client (that I have found)... but that's been my experience.
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If "custom" was mass produced... it would just be a "product line." Custom Processing Unlimited