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[i]Also its not the same as removing the faceplates and using the test socket, doing that you are not touching BTOR's wiring, when you use a fibre coupler on the fibre cable going into the ONT you are altering their wiring, so no its not the same thing.
What you are suggesting to do would be the same thing as re-locating the Master Socket and adding extra cable between the BT80 box and the Master Socket.
Except it's not. Unplugging a simplex SC connector from the ONT and whipping out a simplex SC-SC coupler and a simplex single mode SC-SC patch lead is not physically altering the Openreach cabling. Now if the cable was fusion spliced into the ONT you would have a point, but it's connectorized so basically you don't.
In addition there is zero potential to do any damage upstream. Well not entirely true I am sure if I fired a 10W laser back down the fibre it would do some damage, but we are not talking about doing anything like that.
However if the engineer would be willing to pull the fibre through a range of preinstalled conduit back to my server cupboard (which by the way mostly already exists even though any fibre installation is years away) then that would be fine. My guess is they would not, hence I would either present them with it preinstalled, or just modify it after they left.
Then again I would be looking to hack an SFP based ONT to replace the box BT provide.
I still find it crazy that the fibre is not terminated at a fix point in the wall so that should the length that is going to the ONT get damaged it can easily be replaced by the end user too without bothering Openreach. Everyone is behaving like the fibre optic cable Openreach is using is something special that Joe Blogs can't source themselves. I guess the main issue would be that most single mode fibre is yellow and not particularly attractive, though that would not be applicable to myself.