Can't answer call

Hello. When someone calling me and I have my iPhone upside down and I turn around it to answer the call - there is only black screen and when I press home button/wake button there is normal slide to unlock screen and the phone still ringing, but I can't answer the call.(Same if I unlock it - can't answer, but still ringing) Or if I don't answer it within 30 secons (don't matter if on lock screen or on unlock screen) so the screen is going to turn off - because I have set up turn off screen after 30 seconds - and iPhone still ringing and when I press home button- again slide to unlock and can't answer the call. Someone have same problem? Is it only iOS 9 bug or I have something wrong with my iPhone??

Thanks