That ‘socket’ is ancient and previously had a hard wired phone connected ,one like you see in 70’s TV shows , with a rotary dial , ( search BT 746 telephone if sufficiently interested ) , they don’t have a cord with a ‘plug’ on , these were normally replaced with a ( relatively) modern PST ( plug and socket) type socket , and those have been in existence since the early 1980’s, that gives an idea of how old that is , it’s typical of what was provided in 1960’s and 1970’s housing , supplied and fitted by the developer to give a ‘finished’ look to the wall .
….if the address has had a recent service from Openreach it’s unlikely to have been via that socket , but it’s possible if that address has been with ‘cable’ for the last 30 years , there may never have been a more modern Openreach service point installed, or there was another PST socket somewhere else in the building, possibly removed by a previous occupant if it were surface mounted.
As far as what you should do , if you want BT service, order it , chances are there will not have been any service recent enough to try and remotely start service, so a full installation would be needed, and a new socket provided, it may even be FTTP is available and in that case , even if that were a more modern socket , it still wouldn’t be used as its for a copper pair service, and you would be getting an ONT not a master socket .
