GV has a policy of porting in only cellular numbers. You could buy a cheap Tracfone or similar, port your number there and then to GV. See
http://www.obitalk.com/forum/index.php?topic=2238.0 .
However, think carefully before taking this approach. VoIP service (of any kind) is usually less reliable than a landine -- a failure of your provider, Internet connection, power or equipment will prevent it from working, which might prevent you from getting help in an emergency. Most people who are willing to accept this, have their cell phones as backup.
It may make sense to have an inexpensive prepaid cell phone (look at Page Plus and T-Mobile prepaid). Or, maybe drop your landline package to the minimum, using GV for long distance. Or, your cable provider may have a promotion for their digital phone service.