Here you may find any ISDN related questions, including ISDN/CAPI driver installation and problems
ISDN stands for Integrated Services Digital Network.
You may find a lot info about it on Wikipedia ISDN page
CAPI stands for Common ISDN Application Programming Interface, an API providing access to the ISDN communications protocol. See description on the official CAPI site
Xeepe PBX works with ISDN hadrware which have CAPI 2.0 drivers.