The above info is NOT correct. You do NOT need wifi at all to use GV. We have used it since before G bought it from the startup that began the company about 4 years ago. As a test, I just turned off wifi connection on my BB and easily placed a call to a number I never called before.
We usually use it for outgoing calls only to call intl. numbers. We use GV for incoming which allows us to give people a "local" number in a diff. location which then gets forwarded to our cell.
GV can be used on dumb phones that never have had wifi ability.
GoogleVoice (domestic call forwarding and cheap intl. calls) Use GV to give us a "home" number in a 2nd location
3 T-Mobile lines - Unlimited talk and text. 1G data on each line . $90/mo. We have had no landline in 8 years