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TELEPHONE APPARATUS; TELEPHONE EXCHANGES; TELEPHONE SYSTEMS COMPRISING TELEPHONE SETS, ELECTRICAL CONNECTIONS AND SWITCHES AND ELECTRICAL AND ELECTRONIC CONTROLS THEREFOR; TELEGRAPH SYSTEMS COMPRISING TELEGRAPH INSTRUMENTS, ELECTRICAL CONNECTIONS AND ELECTRICAL AND ELECTRONIC CONTROLS THEREFOR; INTERCOM SYSTEMS COMPRISING SPEAKERS, MICROPHONES, AND ELECTRICAL CONNECTORS THEREFOR; WIRELESS PAGING SYSTEMS COMPRISING RECEIVERS AND TRANSMITTERS; FIRE AND BURGLAR ALARMS COMPRISING DETECTORS AND VISUAL AND AUDIO ALARM SIGNALLING DEVICES