Hi @Nicolas Hadjisavvas -
I apologize for my late response (I received no email notification for your message for some reason..)
First, please make sure that you did not choose Viber as your phone's default dialer. This is a setting of the phone itself, and can be changed back. Check out this thread where this issue is explained:
http://forums.androidcentral.com/verizon-htc-thunderbolt/139889-reset-phone-dialer.html
If it still doesn't work, then we would appreciate if you could take a minute and report the details to our support team using this direct link:
Your ticket details (when you submit the ticket, please make sure to add the URL of our conversation). With this information, our development team will hopefully be able to find the source of the problem, and ultimately resolve it.
Thanks in advance