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Did you try a software repair?
Sorry to hear that your still having this issue.
The issue with the phone displaying wrong battery level was is fixed in 16.0.B.2.9 and later. Please make sure that you have at least this version in your phone. If you have the latest software and still see this issue i recommend that you contact a service location or local Sony Xperia support for assistance: http://www.sonymobile.com/global-en/support/contact-us/.
I haven't checked this on Xperia C but it works like that in for example Z1 Compact as well. When a headset is connected the proximity sensor is not used which has been forwarded internally as feedback.
What you can do is to manually turn off the display using the on/off button but make sure it's not set to end calls under Settings -> Accessibility -> Power button ends calls.
You Are Right Sony Cheat Customers By Selling Defect phone in market as most of the phone are defected & have most of the phone issues & we which wated our money for buying defect phone of sony . we all consumer should log a offical complain to court for miss selling the defect phone in market with fake & lies infomation of products ..