Update to IOS 6 seems to have switched the mute key and volume ringers off completely
go to. this worked for my Iphone 4
Assistive touch switch on.
my iphone on the side is showing mute not on, using the assistive touch ( the white button that appears on your home screen, go to device and swithc the mute key to the opposite. on my phone mute on (bell with cross through it, opposite to phone), my phone now rings.