No idea. But if using corded phones only and no noise then it is a work around.
Everyone is guessing and the issue sounds like it will require a decent engineer to solve the issue.
Thanks to all for input
I have another engineer visit scheduled for Thursday 9th March.
I will push the issue of testing the DECT phone directly on the BT line in th comms room so ruling out all of the non-BT wiring in this place.
Then hopefully will go from there.
Engineer was new into the job by only few months - young man with excellent approach.
His first move was to fit an Openreach adsl splitter plate at the master socket.
Whistle still present.
He then pulled the wires out of the plate that fed to the star wired extensions.
Replaced the wires - with difficulty in those new compression fittings and reversed their polarity.
Whistle stayed gone.
Brilliant - only trouble is I have the router/hub in a broom cupboard with no power socket, but I am sorting that shortly and using powerline adaptors to feed the LAN signal to my TV and BT box location.
Thanks for the suggestions and help guys - just brilliant!