I made 2 VOIP calls today, through my new phone through wifi+DSL connectivity. Currently I could only test out Skype calls, which thankfully went very well, with different software. This is the first time I got to try this out.
Both software, Fring and Nimbuzz, offer great voice services, as per the small experiment I did. The dialout was to a Skype user in my office, who was online at that time. Next step, get a US number and try a direct dialout to my phone, via both software. Should work fine.