I'm not sure, but I'm pretty sure it dials out via the voice plan, because when I was reading up how to get a VOIP client on my phone so I could talk to people via by Bluetooth headset when I was driving and not have to ask them to pay for international rates the directions/explanation involved monitoring a phone number and putting it on the F&F plan...
Anyway, I'm sure there are some that would do it, but you'd probably have to set something up before you left, not that you weren't counting on having to do that anyway.
Were you going to be calling people on cell phones, land lines, or were you looking at basically using your phone as a voice chat mic/speaker to people who would be on their computers/cell phones?
I don't know how it'd work, but you might look into a Skype/VOIP handset and setting up a Skype account. I think they have wireless ones that don't require hooking into a PC, but I honestly don't know if there's a lot more setup than that which would make using it in a foreign country impossible.