“Briefly, they provide a black-box with phone socket on the back:”
If you mean by ‘black box’ a fibre router, then yes this what the fibre ISP will supply but it will not have a ‘phone socket’ on the back if you mean by this the ‘traditional’ BT type socket. It will have an analogue RJ11 socket which will accept an RJ11 to BT socket adapter and you can plug a tone dialling analogue phone into this and get dial tone, provided you have opted to have a ‘land-line’ service with your ISP. But the chances are, especially with an older model of phone, that it will not ring if you receive a call. This is because older phones need the capacitor that is contained in a BT master socket in order to filter off the AC ringing current.
You have to cut the BT socket off the adapter and connect the two central conductors of the ribbon cable (it’s usually a ribbon cable on the adapter) to the IDC (insulation displacement connector) terminals 2 & 5 of the BT master socket. Your phone should now ring. If you are changing your ISP at the same time as having a fibre connection, and wish to retain your landline service and number, this will need to be ‘ported’ over to your new ISP provider just like if you changed your mobile phone provider and kept your original mobile number. The is likely to be a small charge to pay for this operation.
If you have a domestic copper network which is usually a chain of sockets connected in series around the house using terminals 2,3, & 5 from the master to extension sockets, then all you have to do is replace the two old incoming copper landline wires (usually orange and white, but could be black and green) in the back of the master socket with the two central conductors from your RJ11 adapter. Your old copper domestic network and all your existing analogue phones, providing they are tone dialling, will function exactly as they did before subject to REN limitations.
It’s so easy to do, it’s about a 5 minute job in total. You do not need to go to the expense of buying an ATA, a new VOIP phone or re-cabling your home network for ethernet. It’s all built into your new fibre router but it only has one input via the RJ11 socket, hence use your old copper network as described above.