BT sockets & line boxes range. NTE5A & NTE5C sockets are the type commonly used & referred to as "BT master sockets". Secondary sockets are used for slave telephone extensions that are wired from the master socket. PABX are the type of socket that some office telephone system use, they can also be used on domestic wiring & allow a telephone to ring without the ring wire (or bell wire) connected so ADSL & VDSL speeds are not affected.
