This is a simple question but I couldn't find the answer in this website?
Is is possible to call phones without a voice plan just using Google Voice?
You can do this with an app called GrooveIP: https://play.google.com/store/apps/details?id=com.gvoip
I have it on my OG Droid, which is no longer activated on VZW, and use it as my "house phone" with my Google Voice number.
No, Google Voice does not work over data. You can use a VoIP solution or do some complicated things with SIP accounts and clients to make use of your Voice account over data, but this isn't supported by Google or the Voice app.