Install ipppd by entering the following commands in the terminal:
sudo apt update sudo apt install ipppd
Description:
ISDN utilities - PPP daemon
This package provides the specially modified version of the Point to Point Protocol daemon needed to connect to the Internet through an internal ISDN card. It also supports dial-in connections from Windows clients.
Homepage: https://www.isdn4linux.de
Version: 1:3.25+dfsg1-9ubuntu2
Section: universe/net