BT style - 2 Series jack LJ2. 68mm x 68mm x 29mm The 2 Series BT style line jack is a compact surface mounting voice jack with BT style socket. The jack is supplied complete with backbox. The cover and backbox are manufactured in robust white ABS. BT style - 3 Series jack 86mm x 86mm x 29mm (with 21mm backbox) The 3 Series BT style line jack features BT style sockets in an 86mm x 86mm shuttered faceplate. The jack is supplied without backbox. The faceplate is manufactured in robust white ABS. BT style - 4 Series jack86mm x 86mm x 29mm (with 21mm backbox) The 4 Series BT style line jacks feature 2 x BT style sockets in an 86mm x 86mm shuttered faceplate.   The 4 Series has two separate lines. The jacks are supplied without backbox. The faceplates are manufactured in robust white ABS.   PSTN master Used for direct connection to exchange line. Contains: 1.8uF ring capacitor 470KOhm resistor 26A surge arrestor. PBX master Used for connection to PBX extension Contains: 1.8uF ring capacitor Secondary Used where no mastering components are required e.g. extension sockets or for two wire telephones.