TELEPHONE APPARATUS; TELEPHONE EXCHANGES; TELEPHONE SYSTEMS COMPRISING TELEPHONE INSTRUMENTS, ELECTRICAL CONNECTIONS, SWITCHES, KEYBOARDS AND ELECTRICAL AND ELECTRONIC CONTROLS THEREFOR; TELEGRAPH SYSTEMS COMPRISING TELEGRAPH INSTRUMENTS, ELECTRICAL CONNECTIONS AND ELECTRICAL AND ELECTRONIC CONTROLS THEREFOR; INTERCOM SYSTEMS COMPRISING SPEAKERS AND MICROPHONES AND ELECTRICAL CONNECTIONS; WIRELESS PAGING SYSTEMS COMPRISING RECEIVERS AND TRANSMITTERS; ELECTRICAL AND ELECTROMECHANICAL TIME INDI...