When William Blake uses the telephone box in the opening ten minutes, he first uses the phone to call 999, it rings but no answer, so we know the phone works.
He then puts some change in the phone to make a second call but then changes his mind, and puts the receiver back down. At this point the change he put in should have come through the mechanism and into the slot in the bottom. You would hear this for sure as its quite a loud noise, this doesn't happen.