$ sudo apt-get update
$ sudo apt-get install isdnutils-base 

isdnutils-base is installed in your system.

Make ensure the isdnutils-base package were installed using the commands given below,

$ sudo dpkg-query -l | grep isdnutils-base *

You will get with isdnutils-base package name, version, architecture and description in a table

isdnutils-base package basic information: