From time to time you need to run normal POTS telephone over structured cabling, the main issue is that in the UK the pin outs can be different from the rest of the world, so buying a BT plug to RJ45 cable doesn’t always work.
Below is a diagram of the cabling pinouts required for the connection to work. In my configuration I’ve ignored the “bell” wire that is sometimes needed.
Also note that on the other side of the structured cabling (i.e. where you are connecting your phone) you must ensure that you have a modtap (balun) to convert from RJ45 back to the BT socket to plug your phone in. You’ll need a PRIMARY (or MASTER) modtap here which has a ringing capacitor in it, otherwise your phone won’t ring. The SECONDARY modtap is designed for DECT phones that supply their own power and so don’t need the ringing capacitor.