It looks more like 7 its says Openreach MK4 at top , right hand slot has a phone shape underneath it and left one some stars ?
Tried to attach picture but it says file too big?
The Mk4 just means the filter, which just unclips.
These do suffer from intermittent faults.
Can you try connecting things up as shown below, and leave it like that and see if you still get the disconnections. You will need to unclip the front, and use a microfilter plugged into the test socket.
Where do i get the microfilter bit from?
They would normally have been supplied with the home hub 4. BT used to send two microfilters, they may still be in the box that the home hub came in. Similar to this.
as @Keith_Beddoe posted you would have used filters prior to getting the filtered master with 2 outlets you may well have just put the filter in a drawer somewhere just in case needed in future
The bt man who fitted the last socket took all the bits away with him, I know I'm over 65 but I don't have one of them, trust me
unfortunately you need a filter before you can use the test socket so can't test to see if problem internal or external. the type of master you have often can get dislodged very slightly if knocked so check that front is securely clipped in place
What you can do, is to unclip the front, then clip it back on and see if its secure.
This post explains the issue.
If you would prefer to try it with a microfilter, they are readily available at minimal cost. I expect the Openreach person who visited has quite a stock now, I wonder what they do with them, I could guess.