FYI, I solved my problem with some people on forums who had the same problem, with various covers phone. I enclose the message of one of them:
"Problem to win iPhone 5C
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the 18-08-2014 2:46 p.m.
Hello everyone,
So now, after several months of research on the subject, I found the answer to my problem.
After a second visit to the Genius Bar Apple (the Carrousel du Louvre in Paris), I was finally able to identify the source of the problem: the used phone cover. Indeed, after several tests "live" I had to go to the obvious: the brand Muvit cover I used interfered with the touch sensitivity when I had a call or incoming sms (also when I needed to turn off an alarm was ringing).
Materially, this is apparently due to the presence of magnet in the small tab at the end of the game that can fold the cover (I do not know if it is very clear ...).
Apparently, they found this case repeatedly and systematically, it is these Muvit covers that are criminalized. Moreover, no Apple Store is selling this cover brand (buying another cover to the Apple Store in the Louvre I saw the brand Muvit was not referenced).
For 15 days I use my phone with my new cover (I took the Sena brand, which protects as much as my old Muvit), I have no problem with the touch as receiving calls, sms or anything else. "