Wi-Fi calling will enable Google Voice users to reduce phone call roaming charges and have better connection if Wi-Fi is available. Of course, calls have to be made from the Google Voice app.
To make calls over Wi-Fi and mobile data, turn on Wi-Fi calling in Google Voice settings, then make the call from the Google Voice app.
You can apply for Google's Voice Public Beta program for Wi-Fi calling by filling out this form, and Google will contact you after.
You can read more about the beta test here.