Filtered Faceplate's, such as BT's plug and play iPlate (Broadband Accelerator) device, are generally used as a replacement for the removable front cover (faceplate) of a master BT phone line extension box. These are the boxes that typically rest on your skirting board, at floor level, where you plug your phone line in.
Most such devices are designed for use with DSL (e.g. ADSL) style broadband services and fit in with BT's NTE-5 master telephone socket. They are used to filter out a lot of line interference (e.g. ring/bell wire), which can cause broadband connection and performance problems, thus giving you a faster and more reliable service.
Be aware that not all BT sockets are NTE5 and may require a unique and more complicated adapter installation to get the most out of your line. Unless the socket is NTE-5 then we would not recommend installing a faceplate without first possessing a good deal of previous electrical/wiring knowledge.
These faceplates are not to be confused with Microfilters / Splitters, although some faceplates can also split the data and voice line too.
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