Landscape typing for messaging is a MUST in your next update, I hope you release it ASAP.
Some small suggestions:
- Add some emoticons on the message
- Make clicking on the Viber Call icon won't directly going to call the person, I made many missed call because of this, should have make 1 more confirmation screen or message before calling the person >.<
- +1 to previous suggestion, an option to change ringtone and message tone.

Subconsciously, ignore that Call Mobile button.  It's just a shortcut to call him with your telco line.  Either Call Viber or send a message.