So can you see it internally if so is it connected to anything? but go back to the nte and look behind the bt plate bet you've got a bit of ug cable crimped to a bit of Jumper wire (yellow blue Blue yellow) usually used in cabinets which then terminates under the screw terminals A and B (some times the ug feed is a bit short after repeated termination and this is the way around it)before disappearing in the upstairs window sill (where it then ends!!).
If you see any blue/ orange/ green/ brown wires in there are they terminated if not see if you can pull the grey wire out from the window sill.