Install isdnutils-doc by entering the following commands in the terminal:
sudo apt update sudo apt install isdnutils-doc
Description:
ISDN utilities - documentation
This package includes the FAQ and other miscellaneous documentation for the ISDN utilities suite, in English and German.
Homepage: https://www.isdn4linux.de
Version: 1:3.25+dfsg1-9ubuntu2
Section: universe/doc