TBR21
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.[1]
It is somewhat equivalent to the US part 68 standard.
TBR-21 has been replaced by the new ETSI standard ES 203-021[2]
The "advisory notes" (ATAAB notes) are no longer being maintained.