QSIG
QSIG is a peer-to-peer signaling system used in corporate voice networking. Â Internationally, QSIG is known as Private Signaling System No. 1 (PSS1). Â This open standard is based on the ITU-T Q.9XX series of recommendations for basic service and supplementary services. Â Therefore, as well as providing inter-PBX communications, QSIG is compatible with public and private ISDN.
QSIG also has one important mechanism known as Generic Functional Procedures (QSIG GF). Â This mechanism provides a standard method for transporting features transparently across a network.
Source: Â Voice over IP Fundamentals (Second Edition) – Cisco Press