Power button ends up the call

Hi . Its me Talal Ahmad. I am using iphone since 2007. But i always had a problem of privacy that when ever i give my phone to some one else during a voice call i try to press power button so that my phone stays locked when the call is over but when ever i press that button the calls ends. After last update apple gave an option to enable and disable power button ends up the call so i used that opportunity but now i have another issue that i am happy that when ever i lock my phone during voice call it doesn’t end the call but now when it comes to reject the call i am unable to reject to call by double pressing the power button. can you tell me any solution please