My iPhone 5s will not ring or vibrate when I get incoming calls …. I have checked all the buttons and suggestions given without success… my latest attempt is go to General >accessibility >scroll to assistive touch and turn it on ,an icon appear ,small black box with white spot press this other icons appear press device and a bell icon appears ,press bell and a bar appears across it leave it like this the turn off assistive touch .. it then works but only for a while appears to re mute itself ,any suggestions thanks