I hope that was a hard wired Data extension, rather than just plugged into the vDSL socket at the master.
If so, there are a couple of ways you could do this, BUT if you replace the socket upstairs it will need to be either a modular unit so that you can have your Fibre Modem socket (RJ11 as it is currently) and a separate phone socket right next to it,
OR there will need to be enough spare cable that you can route it through 2 separate boxes, the current one for the Fibre Modem and one next to it for the Phone, ideally by extending the Orange pair. Please note that you must connect ANY phone stuff to the Filtered (phone) side of the vDSL filter at the Master socket. That is to the IDC terminals on the rear of the removable faceplate that has the phone socket on it. Whichever, you will need an IDC tool, get one of the cheap plastic ones, quite suitable for what you want if used carefully.
I would recommend the latter method, a totally separate socket. Mainly because if you have any problems BT could get a bit funny if you've re-terminated the Data Extension bit.
You could either fit this socket prior to the Modem socket on the cable run, so you would gently strip back the sheath until you had enough length on the orange wires, or, the way I would do it, is get a couple of jelly crimps and a length of CW1308 cable (standard phone cable as used by BT). If you don't have such items around try and scrounge them off a friendly OpenReach eng. Jelly crimp one end of this bit of cable to the spare Orange pair, I would stick to the same colours throughout, ie Orange/white trace to Orange/white trace and White/orange trace to White/orange trace, the other end would go into your new phone socket. Make sure the wires remain a twisted pair. I'd use the Orange/white trace to terminal 2 and the White/orange to terminal 5.
By choosing a suitable length of cable you can put the socket in the most convenient location, and if it's a cordless base station I would want it as far away as possible form the Modem and Router.
Now as far as the socket itself goes, you should get an LJU type such as the LJU3 Secondary LJU3/3A as shown
on this pageThen when done there, go to your Master socket and remove the front plate and connect the spare Orange pair to the rear of the face plate terminals 2 & 5 as previously mentioned.
There's only one point remaining and that is if the phone you are using requires a bell wire, most modern phones don't. If the phone in your extension doesn't ring, this is because you haven't fed the filtered bell wire from the master socket (nor would I recommend doing so if using the same cable as the vDSL pair - but in any event you don't have a spare wire). The quick solution is to plug an ADSL filter into the extension socket and the phone into that. ADSL filters provide a filtered bell wire.
The only minor downside is that using the spare pair in the cable may give you slightly poorer performance on your Fibre. Mind you, this would be no different to using a separate cable that was running alongside the data cable. The only way to avoid this would be a totally separate cable and route for your phone extension socket from the master (when you could also connect the bell wire).
HTH.
