If I answer a call,the screen goes blank,I can speak normally:After the call even the other person finishes the conversation,I still can't do anything on the phone,because the screen stays blank.If I have another call this will awake the screen and it will work normally,I mean,I can take the call,answer and speak on the phone but with the screen blank again. and so on.To have the phone back to normal,I have to reboot it by switching the phone off and on again. Very strange.This happened after I unlocked the phone ,probably because I left the SIM card in ?!!! Any possible advice/solution? I tried soft and hard/factory reset with no results.