TBR21 is a European telecommunications standard published by ETSI, and is the standard to which all telephone equipment must adhere to, to be allowed connection to Europe's public switched telephone network

Public switched telephone network

The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...

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It is somewhat equivalent to the US part 68

Part 68

In telecommunication, part 68 is the section of Title 47 of the Code of Federal Regulations of the United States governing the direct connection of telecommunications equipment and customer premises wiring with the public switched telephone network and certain private line services, such as ...

standard.

TBR-21 has been replaced by the new ETSI standard ES 203-21

The "advisory notes" (ATAAB notes) are no longer being maintained.