Did a BT bloke (or blokess) test the line from YOUR premises? With the line length being "25m too short" (wonder how they measure to that degree of accuracy over a 3 mile cable route) they should give you the benefit of the doubt and at least send an "engineer" to do a definitive line quality check.
I work quite closely with BT in Scotland and up here they are very pro-active (in my experience). No horribly bad crossing of swords etc.
I'm assuming that you have some association with aircraft, and that it might be with an ATC provider. If you do work for an ATC provider, have a word with one of your ATEs and see if he/she can give you a name in BT that might have some clout and try to get one of their engineers on-site to TEST the line rather than just look up line length records. If you don't work for ATC, have a word with your local ATC unit and chat to their engineers (ATEs).
It's NOT possible to test line loss or noise with the standard BT line jack termination from a remote location or exchange. For that to be possible you need a Line Termination Unit (LTU) which is an active device, installed in you premises, and can be interrogated from one of BT's management centres. We use them on lines that we class as operationally important and have to pay a premium for the privelige.
Hope you have some success.
awra best
ST