The dT Fibre Router will provide a RJ11 port on the back of the Router. It is possible to connect your Telephone handset to this port and you will be able to make & receive calls as usual.
As the Copper wire (PTR) box us usually disabled &/or removed, this means that the copper wire will cease to work from your property to the local exchange. However, the “internal” copper circuits should still work. This means that if you use a traditional telephone cable and connect it from the RJ11 port of the new dT Fibre Router and connect it to one of your “copper” Telefonica sockets on the wall, then this should bring the copper circuit in your property back to life and at the same time any other Telephone handsets should work as usual too.