I am trying to contact someone but whenever I call the answer is it is an incorrect number. Now I know for sure that it is a correct number, so what does this mean

It means that either (a) you have the wrong number or (b) the person answering the phone is lying to you.

How do you know for sure that you have the correct number? Perhaps your source is outdated.

Oh no I have been talking on this number a few days back I know it's the correct no. It is saved in my cell phone with the owner's name