@Bookworms It's just part of the ethernet standard, there are 8-wire and 4-wire cabling standards, so you just wire up the RJ45 connectors to the 4-wire standard (which inconveniently has two variations!). Almost all equipment will work with it and run at 100mbps.
If you look at the short ethernet cables provided with many older routers (definitely Netgear) you'll see just 4 cores wired into the RJ45 plugs.
The BT phone cabling (the striped core sort, not the older solid colour core variety) is twisted pairs to IIRC something like a Cat3 standard. Ethernet works fine over that at domestic distances and 100mbps.
Loads of diagrams and Youtube videos online explaining how to do it. I just repurposed old, redundant phone wiring that was sitting there unused once cordless phones became ubiquitous.
