@DJ: skype is not SIP protocol telephony, they use their own tech
@pantho: fist of all you need to check the firmware of your N80. type *#0000#. if its version is 3.xxx then its fine. Find or install the voip app called Gizmoproject. Make an account there. Adjust your wifi router/settings of your phonea and that is all. Your N80 is ready, and from what I know, Gizmoproject is likely to dethrone skype soon.
edit: you will be using your wifi local access point to call using voip, but take a closer look how this can be done by checking on
www.gizmoproject.com and DM me your SIP so I can call you too