How do I make wi-fi calls from a cell phone with no number now associated with it?
It's not possible to have the same number on two different phones.

On your Nexus S, you could install Skype. That will give you FREE VOIP for Skype-to-Skype calls. You don't need a number for this, just an internet connection (and a Skype account, which is free). Take note, however, Skype to mobile / landline is not free.

For other alternatives, search for voip in Google Play.

Edit #1: I looked around and found IPKall. It gives you a FREE Washington number. Is this the kind of service you are looking for?