While.

While. I haven't made many VoIP calls using Hangouts, I have received a bunch. It usually works pretty good, second only to T-Mobile's baked in WiFi calling feature on my Note II.

I've never used any of the other apps mentioned in this article so I can't compare but I wish they'd tested the BBM VoIP in comparison as well.
http://customstoday.com.pk/google-hangouts-app-is-better-than-viber-wechat-whatsapp-for-making-calls-3/