(I found solution on another discussion by 17beachside):-
This worked for my Iphone 4
Firstly make sure your mute isn't on (the top left slide should NOT show red) Then go to:-
Accessibility (go down under PHYSICAL & MOTOR)
Assistive touch - switch On.
Click on this assistive touch which should now appear on your screen (it's a faded square with a white circle icon button)
Now, select DEVICE and make sure the bell has a line through it (it should say Mute with a line through the bell).
PS. My iphone on the side is showing mute not on (no red), using the assistive touch ( the white button that appears on your home screen, go to DEVICE and switch the mute key to the opposite... that is on my phone showing the MUTE as on (bell with cross through it, opposite to phone), my phone now rings.
Hope this works for you like it worked for me!!