Ever since they did that update to the phone while talking with people if you're face touches the screen it hangs up why?
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make sure that you have it set for it to turn off the screen when you hold it up on a call. i believe it would be in the phone/dialer app settings.
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What phone and what upgrade?
Take a look at your display settings as well, the length of screen timeout can affect this as well as some screen protectors and cases that come too close or too far around the screen edges.