You want estimate what percentage of people in your town own a BMW. You call 100 people at random and ask them whether they own a BMW. Even assuming that everyone answers truthfully, why will you still not know accurately what percentage of people own BMWs?

Question

faiqraees · Answer

Becuase the percentage you will acquire through the results will give you the percentage of those 100 people not the while town.
For example you call 100 people and coincidently they all have BMWs. Does this mean that all those 100 people have BMW cars? Yes.  Does this mean that all  people in TOWN have BMW cars? Certainly not.

mathmale · Answer

Or consider that not everyone has a phone (surprise, surprise!).  You'd not reach poorer residents.  Thus, your sample of 100 residents would not be truly representative of the population of the town as a whole.