In the UK, the ISDN concept was first introduced to customers by BT with their DASS2 connections.
Q.931 is the name of the CCITT document that describes the agreed signalling format for International ISDN.
The organisation set out the internationally agreed standards for telecommunications, and subsequently evolved into the ITU.
Most modern PABXs can handle many different types of signalling system, however the trend seems to be away from the DASS2 (which is no longer being developed by BT and has been known to deny problems with their DASS2 circuits), and towards the internationally recognised Q.931 standard, which is utilised by many countries' telephony service providers.
These cover features that may be instigated by exchanges via the ISDN and require a higher degree of layer 3 implementation.