As part of the BT Technician's investigationsfollowing my reduction in broadband speed following an outside pole replacement, he replaced my old (probably fitted around 1985) master socket with a 5c version, and reconnected my internal extension cable that runs to the extension socket into which my SH2 is plugged.
All has been reasonably well until I plugged a BT telephone plug into the Master Socket socket - this immediately cut the connection to my extension cable and my SH2 started flashing purple.
Is that normal behaviour for a 5c master socket? I can't imagine it is, because it means you can't have both an extension cable and the Master Socket socket in use at the same time
Have you tried using old style filter in your extension as the data extension from 5c master is unfiltered
Also, if you remove the lower faceplate and plug into the test socket behind it, it disconnects any extensions.
I suspect the socket has been wired incorrectly. The extension for the hub should be connected to the data extension terminals.