Voice mail systems are designed to work with a phone system. The term "integration" is used to refer to a high level of phone system and voice mail system interactivity.

In most cases, integration comes down to three features. First, a light on the phone indicates when messages are waiting in the mailbox. This notifies users when to check the system.

A second integration feature is the ability to automatically forward calls to the correct mailbox when a call is not answered. This alleviates the need for the caller to remember the correct mailbox number to leave a message.

A third integration feature is known as "return to operator." This allows a caller to exit the voice mail system and reach a live attendant at any time.

Not all voice mail and phone system combinations can attain this level of integration. Before buying, you need to check that the particular combination you are considering will at least handle these three tasks.