Located just about here on a street which has buried telephone cables, so I assume that it is some form of marker or junction box.
There is a spigot for a utility key, so I assume that it can be opened.
Update 10th Feb
According to a thread on ThinkBroadband, this is probably a jointing post and is likely to still be live and be connecting several lines. May be made of asbestos, so best not to lick it.
Update 12th Feb
I found a clutch of these near the Mill Street Roundabout, about two miles away from the first instance in in the heart of the town rather than the rural outskirts.
Again, there are no overhead telephone cables. These ones were marked PO, which would date them after 1969. All four in this area were marked with four-digit serials and were located near square, double-hatches in the pavement:
Upon the hatch covers were PO inscriptions. Can we date this to 1973? Stay tuned!