WiFi Question

I have a dumb question but I am curious...





When my 3G iPhone is on WiFi, what does the phone use (network-wise) for making voice calls?





I'm guessing that's dependent on what type of network coverage your in (i.e GSM or 3G), but I've never been able to tell. I've noticed, however, that using WiFi even when in an EDGE only coverage area that my phone doesnt interfere with my computer speakers or anything else when making or receiving a voice call...





I know dumb question but I've not been able to figure this out myself