To stop this and revert the phone back to its original state you need to do the following.
Go to Settings>Applications (Manage Applications is another step in some versions of android) and select the Viber application from the list (It can be another Dialer Application that you have downloaded)
When you are in the Chosen Applications settings you should see Clear Defaults, about half way down. Select this and proceed with the on screen prompts.
Once this is done try to make the call again, before it will let you complete the call it will ask you which application you would like to use. Select your phones default Dialer this will be the one with Dialer Icon you are used to tapping. Make sure you check the "Make Default" Box before continuing.