When solving problems with ISDN devices connected to ISPBXs, it is can be useful to discover what standards the ISPBX is using internally, and what services and capabilities the ISPBX supports, and whether these have been enabled for the port in question.

Connecting a device directly to an external ISDN line can be an extremely  useful diagnostic tool in determining whether the problem is with the device or with the way that the ISPBX is programmed to handle calls.

Customarily, most ISDN problems which occur when calls are passing through an ISPBX are resolved by the appropriate adjustment of parameters on the ISPBX itself.