[tialaramex]

Depending on how modern it is, you might well have "telephone extensions". In principle this is Cat 3 cable strung in a tree shape, but in practice the electrician is probably buying wholesale cable - carrying one reel of Cat 5 for all jobs is easier than owning a Cat 3 reel and a Cat 5 reel and bringing the right one for each job, the price is usually either identical or within pennies. So there's an excellent chance it's Cat 5 cable anyway†.

Now, Cat 5 cable is a perfectly good telephone cable, but it's also Gigabit Ethernet (over reasonable distances, you don't live in a mansion). The tree shape won't work for networking, but the individual cables buried in walls or elsewhere are basically just right there already. You just hook the existing cables to new Ethernet shaped faceplates. I am literally writing this from a wired connection in a bedroom, nobody built this to have Ethernet, they built it to put a phone in the main bedroom, but it's 2025, nobody owns a wired phone, everybody needs Internet.

† Also the network cards can't tell, they will try to achieve 1000 Mbit/s and chances are they succeed even if the cable isn't actually rated Category 5. I have retro-fitted modern switches to an ancient building (the old Mountbatten chip fab at the University of Southampton, before it burned down) and in 90+% of cases this "upgrades" the connection to Gigabit because the Cat 3 cable pulled a decade or more earlier was good enough.