Try "Settings" and then "Sound" and then "Volume" There should be options for different types of sounds.
Also, you would need to check the call screen while you are 'in/on' a call to see if the mute there is being selected.
Another way to test is to try and call another phone you have access to. So you can test all the settings while in a call. It could turn out not to be your phone even. Can't know without trying different things.