"Skype was designed as a desktop program, and it’s never fully made itself smartphone-friendly. To be available for a call on Skype, you must sign into the program and keep it running. If you are signed off from Skype, no one can reach you.
Viber is constructed with the smartphone in mind. When you make a Viber call, your request is routed to a central Viber server. The server checks to see if the recipient of the call currently has Viber running in the background on his phone. If so, the phone starts ringing; if not, the recipient’s phone receives a push notification that essentially turns Viber on automatically, causing the phone to ring. Once your friend answers the call, audio is routed to the closest Viber server, ensuring the connection. Viber then attempts to create a direct Internet connection between you and your friend’s phone, rather than using the local Viber server. If you initiate the call through a Wi-Fi network and then move beyond the range of this network—say, walking from your office to your car—Viber will shift your call onto your mobile 3G network. All of this happens without the user noticing it. Viber says it uses high-definition audio, just like MP3, providing better audio quality than is available through GSM or a land line."