I don’t know that number

In my observation people nowadays do not answer their phones if they don’t recognise the number. Most obvious with cell phones. However, also observed (and annoyed by) people don’t cancel the call, they let the phone ring, or squeeze it in their hand in an effort to cover the speaker or shove it in a drawer until it stops ringing. Why is this? If you cancel the call does it not go to the answer phone? If you don’t know the number why do you want to give the person the opportunity to leave a message? If you are going to extend them this courtesy why not just answer the phone instead of being discourteous to everyone around you who has to listen to the ignored phone ring? Maybe the phone is left to ring so that if it was a call that you really should have answered you can  get away with saying “I didn’t hear the phone” or “I was in the bathroom” … </rant>