You have no understanding
Google Voice is NOT a Telecommunication Service, it's a Voice Routing, Recording, and Automation system. Essentially, it's a VRU, a device I can connect to any ordinary line at home or in a business which can receive, manipulate, and redirect calls, but it by itself does not constitute an endpoint telephone.
yes, it supports SIP endpoints like a gizmo client, but you have to have an existing account on gizmo's service, just like having to have a existing phone with a 3rd party provider for a cell phone or landline.
I used to do the exact same thing at home using an Asterisk telephony board and 2 home phone lines and my cell phone. I had a cell plan that let me place and receive calls to home free, and MCI unlimited calling from home to all of america. I put a "child locked" account on my cell phone, with a 100 minute plan only able to send and receive calls from 1 number (the home one), so in essence had a $15 a month fully unlimited cell phone calling plan. I could also record 100% of all calls,or do 3 way calling without surcharges. I also used it as a call screening service. Several years ago a lighting storm took out the board, which was a handmedown from a client who didn't need it any more, and I could never afford to replace it, so I no longer had my home service. Google voice is the same thing, simply hosted by someone else...
You can not connect a SIP or VoIP device directly to google, without a3rd party, and make and receive calls. They are a 3rd party intermediary service. They are NOT a telco.
You should not be writing articles about technology you do not understand.