I've attempted to write a bare bones how to guide for making free SIP voip calls. Details can be found at my blog:
Home - Loganss' Nexus One blog
I've broken up the guides to a beginner, intermediate, and advanced section. If ur comfortable w ur phone I'd recommend the intermediate guide.
If u are abroad and don't have access to a US number in order to verify sip provider account creation for the sip providers I've listed, let me kno and I'll provide more info. When I get time ill add a non-us resident guide.
I've also provided my experiences with different confugurations and some call quality results.