I am connected to the master (master -> vr600 -> pc) and it was an engineer install many many years ago. I would post a picture but not sure how to do that!

There are two connections; one for phone and one for modem. Remove this and get the next level which is the in built filter and then the test socket.

We had BT out for a crackly line about 3 years ago. No faults found internally. The engineer repaired a joint in an inspection pit about 100m from me and then found another fault closer to the cabinet but no inspection pit so he changed the pairs. Crackling line fault gone. However, seem to have strange recurring problem - we go away and voice connection is lost but data is ok. Happened twice around same time of year (different years!). Last time we lost both voice and data. Could be engineer disturbing pairs in the cabinet.

Looks like I will have to grin and bear my problem until my next "proper" fault