I have an elderly friend who has just switched to fibre and seems to have had a pretty ghetto install, a holesaw through the wall to his office with a couple of phone leads joined with a coupler. They'd left his phone plugged into the copper, so he was at a loss when the phone cut out that evening, an extra ethernet lead was going from the router back to the ONT and he didn't have RJ11 leads to plug his phone into the ONT.

I haven't seen his home's telephone wiring, depending on the topology, does he need to cut the external connection (as per this post to use his existing wiring for extensions? Or will having been disconnected from the copper POTS service be enough separation from the wider network?