It is pretty obvious that you would not want to receive a call a special gathering or any other sensitive place. At this juncture, rejecting an incoming call may be seen as the only option.
However, this is not the case with Nokia Lumia. This phone has a feature that can enable you to silence calls without rejecting. It is as simple as pressing the volume button by the side of your phone when there is an incoming call. This mutes the volume of your phone instantly.