My iphone doesn't notify me when I'm receiving a call?

Someone will tell me that they called me, but yet my phone does not ring nor does it give me any type of notification. Not even a missed call. Anyone know what could be wrong with it? It is the iphone 4s & i have had it for about a year and 1/2.

The bar thing at the left top hand corner. Switch that on so you see a bell.