After a while I can not answer the call - screens turns black
Hi,
I am new to iPhone and run into this weird thing. When is someone calling to me, the screens after a while turns black (I guess it is 30 sec - what I have set for locking the screen). However when I turn on the screen again I can not answer the call. Screen is acting like nothing is happening and I can use the phone normaly, but I can still hear my ringtone and can do nothing about it. I can't answer it nor decline it.
I hope I explained it well. Thanks for any advice.
iPhone 6s, iOS 9.2.1