Recommend app to call using wifi

Hey, Anyone recommend an app that allows free wifi calls on a non-active cell phone? Viber and most others allow free calls but only to other users that have the same app installed on their cell, I'm trying to avoid that if that's possible.
I'd even go the sip provider route and then use csipsimple app if the sip provider was free. Any suggestions...just trying to save minutes on my active cell plan
and always being near a wifi connection would make it easy if that's doable...
Thanks...

 

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