Funnily enough, I'd just sketched out this diagram a couple of days ago which Hal will have seen already:
![[Linked Image]](https://www.electrical-contractor.net/PC/UKmodular.jpg)
On the end of the phone cord, the loop/speaking/dialing circuit is across red & white, with the ringer between blue & white. Pin #4 on the jacks is unused on a basic line, but is typically the earth for recall on PBX extensions. 1 & 6 (white/green pair) are only ever used in "special" applications like line seizing for auto dialers and such like.
This wiring arrangement was only introduced in the early 1980s with the opening of the market to privately owned equipment. Prior to that the 1.8uF capacitor was located in the phone, although on multi-phone extension plans (either hardwired or jack connected) there was still another wire interconnecting the phones, plus the ringers were wired in series instead of parallel.
