It is a small building dating from 1970 which occupies the corner between the former Brass Shop and the Old Oil Store, which after the revival became the S&T Stores. However its rendered finish does not match the exposed stone of its neighbours and its low single-pitch roof neither conceals nor matches the gable end of the S&T stores, which it otherwise obscures. It housed the Strowger electro-mechanical telephone exchanges used from 1960 until 2015 which were installed around the Railway and maintained by a dedicated band of volunteers who salvaged and reconditioned equipment the GPO no longer required.
Following the switch-off, much equipment has been disposed of, some fetching useful sums for FRS projects. The plan was to clear out the buildings for repairs and future use. See here for a 1969 picture before the extension was added, or below for the scene in 1962.
The plan is now to remove the Telephone building to reveal the old Oil Store/S&T Store and use it as an IT room for the site, following renovation. This is part of the Boston Lodge Interpretation and Development Project.
The Old Oil Store building is described by CADW a 'Former Oil Store' (14418) and listed grade 2.