Thanks. Haven’t seen that before. FVA notation suggests it was used at the time Openreach ONTs still spotted FVA voice ports. So probably something the engineer had on the van.

As it’s fitted over the master socket, as said above, it should then work to isolate the incoming copper pair from your extension wiring when in the FVA position. You can of course easily check if dial tone disappears (with nothing connected to the front of the faceplate).

Then it’s a matter of connecting your ATA voice port into the front of the faceplate to re-inject voice to the other extensions when the switch is in the FVA position. You should then get dial tone on the extensions courtesy of your ATA.

As suggested, the engineer has fitted one of the old FVA NTE faceplates. You’d fit a PSTN jack on to a two pair internal wire., this connected to the FVA voice port on the ONT.
You ran the wire into the FVA front plate and terminated on the lugs on the back.

You could then use the switch on the side to isolate what fed any existing extensions.

The idea was that your FVA dial tone wasn’t being sent back down the old copper pair BUT if a (for instance) new homeowner wanted FTTC rather than FTTP, they could be requested to move the switch and allow service that way.

Things have moved on a great deal in the last 5 to 10 years.

Things have moved on a great deal in the last 5 to 10 years.

Yep sometime though in circles or circles within circles 🤣.

Related thread from almost 7 years ago…FVA Installation and extension sockets