Ifound the soloutin of this problem , the problem is sony have some probem in device upgrade ( not happening for every one) in your case and mine we have the same issue, the problem is random it happens in call answering it happen when u play game and etc but the fact is it's not going away, and the worst part is later on my touch screen didn't even function well after those issues, BUT i found the soloutin for that simply ireset the phone by taking simcard out to reset the phone or just by holding power button and volume up, but the wasnt the permanent soloutin.
after trying soooooo many things ifound the problem it is a Software issue and not hardware ( atleast in most of the cases) in order to fix it :
Soloutin :
U need a clean instal, Install sony pc or mac program, take a full backup from your files and data, after on on left up corner of the program select the link it says your phone or tablet is up to date
Then select repair my phone /tablet, its easy from now on u need atleast 5gb free space on your os drive after it program ask are u sure u wanna flash the phone ( this is not hard resseting simply lets say ptogram wipe youre phone download your latest android os from internet and it will upgrade it on ure phone lets call it a Clean instal) it will gonna take around 1 hour so be patient. After this u will have a phone witout any kind of problem so remember this is not the same with hard reset.
Hope this help to you and everyone else with same isuue.
