Abstract: A telecommunications security device for use on the communication medium includes a first and a second security unit each arranged to be inserted into for example the telephone line adjacent a user device. The units are identical and therefore either can act as a central unit for example for a computer access port with the other providing one of a set of remote units. Each unit includes a separable memory module with all the modules having a memory storing identical information. The information stored includes a plurality of pairs of random signals one of each pair providing a request signal and the other the security code. The central unit on receipt of a telephone call provides a signal requesting an ID code from the remote unit and on receipt of the ID code issues from one of the pairs the security code request signal. On matching the received code with the expected code a transmission gate is opened.