There are two ways: Persistent & Per Call

Calling Line ID Blocking Persistent
This service enables a user to persistently block delivery of his or her identity to the called party.

Details:
Calling Line ID Blocking is used to block or allow the delivery of a user's identity (both name and number) to a called party. When active, calls made by the user to parties outside of the enterprise have the presentation of their identity (name and number) blocked. The blocking is achieved by setting the presentation indicator associated with the calling party number to "private", which prevents the user's identity from being presented to the called party's device.

Setup:
The user configures the service activation and deactivation via the personal web portal, or by dialing a configurable feature access code from their phone.

Web Portal:
Click Outgoing Calls. Click Line ID Blocking. Click On (or off.) Click OK.

Feature Access Code:
To enable Blocking (hide caller id) dial *43# and disabled (show caller id) by dialing #43#.

Calling Line ID Blocking Per Call
This service overrides the persistent blocking of the calling line ID (CLID) so users can block the delivery of their identities for the next call. At the end of the call, the presentation of the user's identity is restored to its persistent status.

Details:
The users block the delivery of their identities for the next call by dialing *67, from their device, before making the call. This results in a confirmation tone followed by a dial tone. The users can then make the call as usual and their identities are blocked. Per-call Calling Line ID Blocking is deactivated automatically when the user hangs up.

Tech Notes:
If the user's identity is persistently blocked, the service is used the same way but has no impact.