FTTC (Fibre to the Cabinet) is a hybrid technology used by the national BT network to improve speed and reliability to your premises.
A high speed fibre optic cable is run from the telephone exchange to your nearest street cabinet, at which point the line is converted to fit on your copper phone line to your building.
BT Openreach will need to change the socket inside as shown at the bottom of the picture.
They will also fit a white modem (at the top of the photo), which requires mains power. This box has two connections, one to the outside world via the phone socket, and one to your Internet router. The modem is wall mountable if you wish and we have seen some suffer from overheating, so don’t cover it up or tuck it away too much.