Describes requirements for series apparatus for connection, in series with a subscriber's main apparatus such as a telephone instrument or a data modem, to certain public switched telephone networks. Apparatus that performs a call routing function is not included. Does not apply to apparatus for connection to shared service lines or 1 + 1 carrier systems or as extensions on any forms of PBX or between the TJF of a PBX and the PSTN lines of the PBX. Includes abbreviations and definitions, general operating and test conditions, marking, information to be provided by the supplier, electrical connection methods and electrical characteristics.