Sorry for the jargon. It’s difficult to explain things quickly without referring to it. Let me try and pad things out a bit:
FTTP is “Fibre To The Premises”, i.e. “Full Fibre”. (The older type of fibre was FTTC, “Fibre To The Cabinet”. This used the old copper phone line from the street cabinet to your door and delivered the broadband through the phone socket. It was also known as just “Fibre”). You indicate that your question is in connection with an upgrade to full fibre, (so you are getting FTTP).
With FTTP, an ONT or Optical Network Termination, is needed to convert the light signals in the fibre to electrical signals that your router can use. It’s the optical equivalent of a modem. It is a small box mounted on the wall, with lights on it, and consequently needs to be close to a power point. This delivers the broadband, (and Digital Voice), and the broadband does not come through the phone socket any more. The phone socket is redundant.
Cat 5/Cat 5e/Cat 6 refers to the performance specification of ethernet cable. Category or “Cat” 5e and 6 are needed for gigabit connections in modern computer networks. Cat 5 is obsolete, (but I believe it can sometimes be used for phone extensions). Most phone extensions are the even older Cat 3, with just 2 twisted pairs of wires, rather than the 4 pairs computers need.
Phone cables are wired in a different pattern to network cables. Modern phones extensions are wired as a daisy chain, one after the other. Computer networks use a star pattern, radiating from a central point. Unfortunately, this means the two wiring installations are physically different even if it is ethernet cable and it will likely not work as a computer network.
