This is also what happens when you pick up the phone to place a call and someone is on the other end (as in they were actually trying to call you, not interrupting someone else's call-in-progress on an extension of the same line). Usually you do not realize this until the person calling you starts shouting "Hello!?" in his/her bewilderment of hearing the tones when you began dialing. If you frequently pick up the phone without it ringing and there is someone on the other end, either you are probably psychic - or you get a LOT of phone calls.