As far as I know, there isn't any VOIP software installed on the Expo. You have to use an app like Skype or Google Voice.
Skype has been discontinued for Windows Phones, but there are still cabs out there. I have a cab, it's not the most up to date version, but it should still work. I can't upload it here, it's too big. If you want it I can probably send it through PM
I personally like Google Voice, I can use it on my land line and mobile, and make long distance calls without charge. It's still in beta, you have to Google - "Google Voice" and ask for an invite, once you get the invite, you select your google number.
There is an app on XDA that makes it work for your phone over WiFihttp://forum.xda-developers.com/showthread.php?p=5855672#post5855672