If your extension can be reached by dialling multiple phone numbers, then you can manage your caller ID preferences by logging into the End User Portal and navigating to the Caller-ID option beneath the Preferences tab.

caller ID preferences

From this interface, you can select which phone number you want to use as your outgoing caller-id from the Outgoing Caller ID drop-down list. This is the phone number that other people will see when you call them.

When you are done, click the Save button to save your settings.