These posts are often found on housing estates built in the sixties or seventies where the telephone wires are fed underground into each house. In this image the bottom of the post is obscured by a modern plastic cable joint container, but it has a similar triangular retaining screw like its larger brother below.
This is a rare wider version of the more common post shown above. Within the jointing post will be a cable joint of thin multi-coloured wires joined together with crimp on insulation displacement grease filled sleeves.
The narrow version are often used as distribution points (DP). A single multi pair cable arriving from a street cabinet is cross connected to thin two pair cables running to each house. This allow economic use of expensive underground cables as only premises requiring a telephone need a corresponding pair of wires to the street cabinet. There will be more outgoing pairs than incoming ones.
The street cabinet provides similar flexibility between thick often multi-100 pair cables to the exchanges allowing them to be cross connected to active pairs in the cable to the DP, whether that be a jointing post DP or pole top DP. Typically there are many more pairs of wires leaving a cabinet as not all will be in use, than arriving from the exchange.