By default, when a phone call is forwarded to an agent’s external phone, the number displayed on the agent’s device is one of the account’s phone number. If the Customer Caller ID option is enabled, the number displayed on the agent’s external phone will then be the contact person’s explicit caller ID. This way, the agent will always know who is calling.
You can enable this feature for your account or a specific phone number. Learn to do so by visiting the articles Routing Settings and Numbers Custom Settings.