no noise, well maybe a TINY amount of static but it is cordless phone and arent those noise margins way too high?
Your noise margins are higher that expected especially up margin but that would actually help to stabilise your line if noise problem
as you are using test socket with new filter and quiet line test is ok then you need to phone customer services and get engineer visit as i cannot see what more you can do
did you try a factory reset of the hub using recessed button on the back?
no i havent tried a factory reset, how long should i keep it connected to the test socket, it hasnt disconnected since putting it in that socket
I would leave it there and see if it remains stable and as you only have the 1 socket it won't affect any other sockets