Hi there, are you trying to change your outbound caller ID? I accomplished this on our system by setting the "From:User Part" variable to "Custom Field" and leaving the Custom Value empty. This causes our CID to appear on external phones as "Unknown". We are using NexVortex, so it may be different for another VoIP provider.
These settings can be found on the "Edit VoIP Provider" screen, in the Outbound Parameters tab. After changing to the custom value, be sure to click the Add/Update button, then the Apply button. Otherwise, the settings will not save.