The ISPBX acts as if it has both a network and a user inside. 

The real (physical) users connect with the network inside the ISPBX over their D channels. If they need to communicate with the public ISDN network (for example, to ring home), the "virtual" user inside the ISPBX sends the service request to the public network on their behalf.

The services provided by the ISPBX are often different from those provided by the public network. This can cause problems!

The network service request that a user sends on his D channel arrive in the ISPBX's internal ISDN. This is not passed directly to the public ISDN. The ISPBX interprets the request and decides what to send on the D channel that connects it to the public ISDN network.