I think that @pippincp could be right,
I had a look on a site called CodeLook and Cabs 14/15 (the nearest to the property is 15) only went live in Jan 2018.
They are part of a Strategic Economic Plan.
There appears to be a department/group called Super Fast Dorset, for now I would suggest contacting them and seeing if they have any information regarding this new cabinet. Also you can check https://www.openreach.co.uk/orpg/home/index.do using the post code or number and it may give more info as well.
I will drop you a Private Message (PM) with one other suggestion, so as to not break any community rules. Look for the Envelope icon on the top right hand side of the community page.
Having worked for Openreach you know better than I which direction to take. Is it an infill cab? I haven't a clue but I know it's a possibility.