Screen blacks out during phone call won't turn on

[Mr.Gadget]

Shouldn't this be considered a defect? (I'm nervous about dismantling my phone).
Also, why does it only happen while on a call and not always? Unless/until the other party hangs up, my phone is useless. Argh

 

[Valarie 41]

After reading responses on here I cleaned the sensors on the front of my phone an it totally worked. Guessing makeup buildup was making my phone think it was still against my face an licking the screen to keep me from pressing buttons.

 

[Dev Dixit]

My screen is not opening when the call is coming from anyone when I push the unlock button then showing the call what should I do