At one stage BTNRs – BT Network Requirments defined the complete Network, e.g. BTNR 167 for NUP.
You will probably need the DASS2 –
Digital Access Signalling System -specification as well.
Most Networks are now using Q.Sig
(ISDN ITU-T Q.931).
Also V5.1 and V5.2 maybe of interest.
Please let me know if you need any inputs. As highlighted, for DASS2, DPNSS1, NUP, etc.. you will need to procure the specifications directly from BT.
VBR/ Wallis Dudhnath