If I were looking for a VoIP app, I would simply search "VoIP" instead of clicking tabs. I'm not sure why it's all the way at 435 in the category link, or how that's calculated. I do know, that if I go to Google Play and search "VoIP", I see VoX Mobile at #1.
Simply: The #1 (searched by "VoIP") app in the Google Play store is VoX Mobile... Party On.