That's only partially true. There are several exceptions one of which is attached and clearly doesn't require it to be announced. This is why it should be an option to enable or disable as the business needs.
"Business telephone" exception
The "business telephone" exception, which generally allows monitoring of calls and taping over an extension phone which is both provided to a subscriber in the ordinary course of a telephone company's business and is being used by that subscriber in the ordinary course of its business. This provision generally permits businesses to monitor the conversations of their employees, including personal conversations.