Why is the person you are calling not answering their phone?

Why is the person you are calling not answering their phone?
your phone is not working properly,their phone is not working properly,they have it on mute,it is switched off,they are in a different room, building or area to their phone,they are having an intense conversation and don't wish to interrupt,they are in the bath/shower,they are asleep,they are having sex,they dropped the phone into the bath/teacup/etc.,the phone has slipped down the back of a sofa,they are listening to their music player with headphones on,they are high on drugs/drinkthey are in a diabetic coma/accident/fuelling up the car/playing soccerthey just can't be bothered,they are afraid of you,they don't like you,they are dead