It is possible to do this for free, I did it once. There are a few voip providers out there that offer a free DID, usually in some area code with a high terminantion fee. The one I found offered free incoming calls and paygo outgoing. Perfect! I used the softphone but if I recall they allowed hardware devices as well. I forwarded GV to this for a time but decided it was just easier to let GV forward to my cell and a routing scheme in the office phone system.
I've also read about guys building up an Asterisk PBX and configuring it in such a way to set up calls through Google Voice. Seems like an awful lot of work when you can get an unlimited cell phone for 40 bucks.
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The google phone can be used by us radio people just have to have a lot of time to set it up. I along with a friend of mine have it setup as well as mix/minus to where we use 2 mixers and 2 sound cards and a compressor. It is a lot of wires but it can be done kids. I think I will take a picture of my studio and show it off here. But it works very clean and just like if I had a phone pot on a studio PC.