Which statement best describes ISDN DSS1?

The statement that describes ISDN DSS1 accurately indicates that it establishes signaling between the customer premises and the central office (CO). ISDN (Integrated Services Digital Network) DSS1 (Digital Subscriber Signaling System No. 1) is primarily used for signaling communication and control. It provides the means through which end devices, such as telephone systems, can communicate with the network, facilitating the setup and management of calls. This signaling protocol handles aspects like call establishment, maintenance, and teardown, ensuring that voice and data calls can be made reliably over the digital network.

In the context of ISDN, signaling is crucial for routing calls efficiently and managing the resources of the network. It does not inherently focus on merely connecting PBXs, transferring data exclusively, or managing phone number portability, which are functionalities associated with other aspects of telecommunications and networking technology.