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Try to clear defaults once for the phone app or reset app preferences. Set a default again. If still having problems,you have an app acting up.
So this occurs from the default messages application? I checked on Z1 and it takes me directly to dialer.
You should contact sony:
http://www.sonymobile.com/global-en/support/contact-us/
I do not have viber,maybe that is why for me it doesn't occur. So if you try to set dialer as always,how is it?
As I have already told you,contact sony:
http://www.sonymobile.com/global-en/support/contact-us/
Sorry to hear that you have issues selecting default dialer.
This issue will be fixed in the update to Android 5.