I've searched Skypes knowledge base and this forum & can't seem to find this topic.

I have just downloaded Skype for iPhone into my iPhone 5. I have wireless access in my home: which does Skype choose by default to carry voice calls, cellular or wireless network (or wifi when in a hotspot)? The reason I ask is that my carrier, Sprint, has rather poor service where I live and Im looking for a way (other than a Sprint Airave system) to have a backup calling mode when the Sprint signal is too weak. Or does Skype use wireless network/wifi for calling at all?