iPhone 6 plus screen black and unresponsive during phone call

I have been experiencing some problems with my iPhone 6 plus (64 GB, space grey) factory unlocked phone, iOS 8.4.1 for the past week.

For the 4th time this week, after making or receiving a normal phone call without the use of a earphone, the screen was still completely black. I couldn't even hang up. I just had to wait and hope the other caller hangs up.

I know of the ambient light sensor at the top of my phone is supposed to turn the screen black when I'm making/receiving a call, but when I'm done the screen is supposed to be back to normal, in these cases the screen was still black after I had removed the phone from my cheek. Pressing the home screen several times did not help, even tried to restart it several times by holding down both the home and power button it still did not work. Only solution was to desperately wait for the other caller to hang up. This has happened 4 times in the last 7 days! I use a normal phone case and screen protector (rubber, not glass). I took off the phone case last time this happened but the screen was still black until the other caller hanged up.

This is extremely frustrating and a poor and disgraceful problem for the iPhone 6 plus. I'm completely appalled by this and I desperately need some help. I'm afraid to answer or make calls if I don't have earphones now!

Has anybody else experienced this? And any advise or help please? Shame on you Apple you should be doing better.