You can find a SIP provider (Flowroute, SIP.US, SIPSTATION.com, Twilio, OnSIP, and many more) that will allow you to either use a desk phone or a software-based phone with a telephone number of your choosing. It's never free, and you can expect to pay $0.009 - $0.02 per-minute for all calls.

Disclaimer: I work for one of the companies that I listed in parentheses.

Depends a bit on which specific workflows you're using Google Voice to enable, but as someone not living in America (so don't have access to Google Voice), I can give you a succinct answer to that: No.

If I want to be able to answer phone calls on my PC, my only options are either an SIP softphone, or a Mac/iPhone using Continuity.