IPhone 5S flashes during phone call

[terryh215]

When I'm on a phone call the screen flashes on and off and stays off most of the time. I can't end the call or click the speakerphone option. What's causing this and how do I fix this?

 

[kch50428]

Could reinstall the iOS via iTunes... and if the issue persists and is easily replicated - going to an iTunes Genius Bar would let you know what options are... warranty replacement, AppleCare...

 

[IAmSixNine]

Has the screen been replaced? Sounds like its the proximity sensor either malfunctioning or being covered / blocked. Do you have a case on your iPhone?