It is not possible to make it work via WiFi because the phone dialer will always use the cellular network. Your WiFi network is not connected to a Public Mobile Telephone System so it cannot interconnect you with the other cellular user.
What you can do is to use a Pocket PC video calling software. But on the other side they must also be using the same software as you, or a compatible one. I never tested but "http://fring.en.softonic.com/pocketpc" claims to support video.