BT junction box with built in REIN filter. The built in filter is designed to reduce induced electrical noise which can not only affect telephones but also broadband modems/routers. This often results in improved syc speeds due to less line disconnects which with VDSL = improved speeds! (ADSL lines can also benefit from this filter but you may need to contact your ISP to request a reset of the line so re-training can commence).
Simply install upstream of the master socket using the screw & IDC terminals (IDC tool also available). If required the filter can be installed immediately AFTER the master socket if the subsequent extension is then feeding the modem/router.
The filter (inductor) in this junction box is the same as the one fitted to the (now discontinued!)