Install isdnutils-base by entering the following commands in the terminal:
sudo apt update sudo apt install isdnutils-base
Description:
ISDN utilities - minimal set
This package provides the basic set of ISDN utilities needed to make use of an ISDN card.
Homepage: https://www.isdn4linux.de
Version: 1:3.25+dfsg1-9ubuntu2
Section: universe/utils