Hi @Shanujan, let me see if I can help out here!
Have you been able to reboot your phone in safe mode to see if the same behaviour occur there? Safe mode is a troubleshooting mode where all downloaded apps are temporarily deactivated, this will show whether the behaviour that you mention is caused by a downloaded app or not. For more information on safe mode and how to use it, click here.
If the behaviour persists in safe mode, the best thing to do is to repair the software on the phone using a computer and Xperia Companion. Keep in mind though that this will erase all user data from the phone, so remember to do a backup before the repair to avoid data loss.
If none of these measures manages to improve the functionality of the phone, I would suggest that you get in contact with your local support team to have your phone sent to an authorized service center. If you have any questions regarding the service such as routines, warranty and consumer laws, these kind of questions are best answered by your local support team as well, as they vary between markets.
I hope that helped!
