A normal PABX (Private Automatic Branch Exchange) handles only speech calls, whereas an Integrated Services Private Branch Exchange (ISPBX) is connected to the ISDN and itself provides ISDN extension lines. 

The ISPBX has the ability to switch incoming calls from outside directly to its extensions. Being digital, it also has the ability to route not only speech calls but data, video, high quality audio, and Group 4 Fax. These calls can also be connected internally from extension to extension.

There are special characteristics about the relationship between individual users, the ISPBX, and the public ISDN. Understanding this relationship is essential when attempting to diagnose problems which can arise when using ISDN devices connected through an ISPBX instead of directly to an public ISDN line.