We integrate Viber and SIP Server which can route Viber enabled calls through Voip Gateway. So, you forward call from SIP to Viber app.
Also DID from any countries forward regular call to Viber.
Look like free roaming.
Now we integrate WebRTC and Viber.
Web based Service "CLick -to-Call" can call to your Viber app.
To make sure it works try our demo-test call to Viber:
http://sipiko.net/virtual_viber_rus.html
http://sipiko.net/virtual_viber_en.html