Interesting. Not sure I've posted this here, maybe on another similar forum, but I run 100mbit Ethernet over my phone wiring WITH NO ADDED EQUIPMENT!
Good old BT basic extension cabling (white/orange/blue not the really old solid coloured sort) is 4-core (2 twisted pairs) and IIRC is roughly Cat3.
Just crimp an RJ45 plug on the end, correctly wired, and off you go! Perfectly good for 100mbits at domestic distances and very flexible, thin, unobtrusive and easy to run.

Many old router cables, if you look carefully, only use 4-core cable. I imagine if you used two cables, or had 8-core, you could probably get Gigabit, but I've never tried and have no need.
My longest run is from the router near the front door to the TV in the lounge on the very opposite side of the house, via skirtings, door frames, architraves etc. and is probably about 25 metres. It works perfectly.

Incidentally SWMBO is totally unaware of this, she just sees an old phone extension cable in some places and doesn't question it.