Setting your call preference to Internet instead of cellular means that when possible your phone will first try to connect any call you make over the Internet. If for any reason a call cannot be completed over the Internet the phone will then try the cellular network. Default Internet calling will only be attempted when you are logged into "Internet telephone" and therefore have a wireless network connection established.

Press the Menu key

Go to "Tools"

Select "Settings"

Click "Call"

Scroll and select "Default call type", change to "Internet"