ISDN behind PABX doesn't work

The only thing you must pay attention with ISDN is that you must connect the eWON to a S0 interface
 (The ISDN BRI interface complies to the S bus I.430 specification).

And, generally, the eWON works well when it is connected DIRECTLY on the ISDN interface from the Provider (France Telecom, Belgacom, Deutsche Telecom) because they use S0 interface.
But it doesn't work when the eWON is connected behind a PABX, because the PABX use another Interface than the S0.