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normally all your internal connections would come from the master socket whereas in your case you have extensions coming from the outside socket where the openreach incoming wire meets your internal connection. this is star wiring and causes problems with broadband you need to engineer to rewire the external master to get rid of star wiring
in normal connections if you take front plate of the master and connect to the test socket which you can now see you should only get a dial tone at that socket and any other socket should be 'dead' in star wiring the sockets would still have a dial tone
if the engineer made the correct changes to the outside master then yes if you remove the front on the MK5c then there should be no dial tone at any extension socket
great - thanks for update