A red button just above the volume button

Are you referring to the physical volume button(s) located on the LEFT SIDE of your iPhone? There is a SILENT SWITCH above that which shows red when it is Switched to the ON position and the iPhone enters the SILENT MODE.

If you are referring to different volume button, then please clarify. Thanks.

Axel F.

That is the silence switch to stop all (or most) sounds

It moves back and forth

(I tape mine up because it turns off in my pocket)