It is called a "stump post", something vey popular along the ECML years ago.
Simply put, it is a post into which can be plugged technicians telephones. These then operate over a special dedicated maintenance circuit in the PSB meaning that the Tech can talk to the Signalman without having to go to a lineside SPT.
These posts also provide convenient test locations which can be used to test out the cabling to search for faults.
During accidents, they could also be used to provide site telephone communication.
They have been taken out progressively over the years. Not sure if your example is still operational or whether it would be out of use.