Signalmen at adjacent signal boxes largely communicate with each other using bell signals. Compared to voice messages, bell signals have the advantage that they are quicker to send and have less risk of being misunderstood. In the early days of railways, they also had the advantage that the equipment they use is simpler than a telephone.
The exact type of equipment used varies between signal boxes according to the signalling regulations in use. However, in general, each signal box will have a bell for each signal box that it sends trains to or receives them from, and some way to ring the equivalent bell in each far signal box. If a signal box uses token instruments, there will be a bell plunger built into each instrument. Other signalling regulations might use a 'tapper' built into a block instrument, or a separate tapper on its own.
In each case, pressing the tapper or plunger will ring a bell in the other signal box. 'Single-stroke' bells are used, so one press of the tapper causes a single 'ding' in the other signal box. The bells in each signal box will be made up of a variety of bells of different pitch and tone, so that the signalman can easily identify each bell by its sound.
It is often possible to hear at least some of a signal box's bells when standing on the platform. Remember, though, that you will nearly always only ever hear the bell signals being received. When a signalman sends a bell signal, his own bells do not ring.