When the handset of a public-network telephone is lifted, the circuit between telephone and switching centre known as the local circuit is immediately barred to potential incoming callers and is said to be engaged or busy. Even though a connection to the intended destination has yet to be established and no information in the form of speech can be exchanged, to all intents and purposes the local circuit is already carrying traffic since it cannot be used for any other call. The initial purpose of the local circuit is to convey signals representing the dialled or keyed digits to the switching centre, so that they may be interpreted in order to extend the path for speech communication to the required destination. Only when a complete end-to-end circuit has been esta blished between the calling and called telephones can the local circuit be used for its intended purpose of exchanging information in the form of speech the originally introduced concept of what is meant by telecommunications traffic. The sounds of speech are represented by variations imposed on a steady electric current, and it is these current variations which are conveyed as a continuous signal between the telephones to provide two-way communication.