it's very easy to config an "S0-Bus" for ISDN modem / fax devices.
First of all, setup a BRI port as "internal"-Bus (example port 2):
Then, for incoming calls, assign a number as "MSN" number (i.e. DID 959595 assigned to MSN 47):
If you need also to dial out from internal ISDN modem / fax, config and assign to internal ISDN port this dialplan:
Note: "Einstellen ITC unrestricted" rule only needed for data calls (modem).