You can do this by using the "External Phone Number Mask" field under the Directory number settings.
For all help desk phones, set the External Phone Number Mask to be the helpdesk number.
Make sure that you check the "Use Calling Party's External Phone Number Mask" in the Route pattern for PSTN. Also if you are using a Route List, make sure that the Use Calling Party's External Phone number mask is set to Default.
Also make sure that the Caller ID DN is not configured in the gateway settings. This would override all other settings.
Depending on how the system is currently setup, this might need some planning to accomodate the calling party number display for normal phones and helpdesk phones.
HTH