By default the PABX sends your extension number as the caller ID. The ISDN network, or other far-end service provider will then try to resolve this to a “valid” number. This resolution may vary between providers, so the results will vary.
To send a different number or to bar the sending of caller ID, edit the caller id override field in the edit-extension screen of the PABX web interface.
Extensive per-provider caller-ID configuration options are available. Click the ‘123’ icon next to the provider that the call is being sent to to check these settings. Different providers have different requirements, and behave differently if invalid numbers are sent to them.