How do I make wi-fi calls from a cell phone with no number now associated with it?

I recently got tired of paying $65+ a month for cell service when I get mostly email. So I dropped my Sprint service in favor of $20 a month, no-contract phone and a Nexus 7, because WiFi is seemingly everywhere. It works great! I have all the service I need, and it's much cheaper. I still have this nice cell phone, a Nexus S. I enabled the WiFi on it. I'd like to make free WiFi calls on it but it no longer has a number associated with it. I ported that number over to my $20 a month phone. Is there anyway I can get a number associated with this old phone so I can sign up for a free voip service? Is there anyway I can make voip calls without a number? Thanks.