Which ISDN protocol allows signaling to be passed between networked PBXs?

The correct choice is QSIG, which is a signaling protocol specifically designed for use in communicating between private branch exchanges (PBXs). It enables the exchange of calls and signaling information between different PBXs, allowing for integrated communication systems and seamless connectivity across various platforms.

QSIG facilitates advanced features such as call transfer, call hold, and conference calls between PBXs of different manufacturers, making it a crucial protocol for organizations that operate with multiple communication systems. Its capabilities extend beyond simple call establishment to support more sophisticated telecommunication features necessary for modern business environments.

Other signaling protocols listed serve different purposes. ISDN BRI (Basic Rate Interface) is primarily designed for connecting end-user devices to the ISDN network but does not specifically handle signaling between PBXs. SS7 (Signaling System No. 7) is used in the public switched telephone network for setting up and managing calls but is not typically utilized directly between individual PBXs. Q.931 is a part of the ISDN standards, focusing on the call control signaling protocol, but does not provide the extensive inter-PBX capabilities that QSIG does, making QSIG the most appropriate answer for the context of signaling between networked PBXs.