Let’s compare Skype and Viber. Both are P2P, Viber is beginning while skype is a mature product. Skype signaling and codec are both proven as super scalable. VIber  supports push message while skype doesn’t for incoming call. Viber is only on iPhone but skype in on Pc and phones. It’s easier to reach people who have Viber app installed, not having to worry about being logged in or having the app running in the background.  Skype is said to have better call audio quality. Both are P2P, Viber is beginning while skype is a mature product. Skype signaling and codec are both proven as super scalable. Viber  supports push message while skype doesn’t for incoming call. Viber is only on iPhone but skype in on Pc and phones skype vs Viber