Which ISDN protocol is used for signaling between networked PBXs?

The signaling protocol used for communication between networked Private Branch Exchanges (PBXs) is QSIG. This protocol is specifically designed for interconnecting these systems, allowing them to exchange call control and signaling information effectively. QSIG supports various advanced telecommunication features, making it suitable for businesses that operate multiple PBXs needing seamless communication.

The other choices serve different purposes; for instance, ISUP (Integrated Services Digital Network User Part) is primarily used for signaling in circuit-switched networks, specifically for establishing and managing telephone calls. SS7 (Signaling System No. 7) is a set of protocols for exchange signaling between elements in the public switched telephone network and, while it can be involved in overall telecommunications signaling, it is not specifically tailored for PBX-to-PBX communication. BSSAP (Base Station Subsystem Application Part) is primarily related to mobile communication networks and does not apply to signaling between PBXs.

Thus, QSIG is distinctly pivotal in facilitating effective signaling between networked PBXs.