Do Not Disturb prevents your phone from ringing when calls come in. Your phone is effectively removed from ring groups, or in a simpler case all your calls are directed to your Message Center.
To enable dial *78
To disable dial *79
A device feature operates only on the specific device. For example, if you had more than one phone in use on your extension (a home phone and a work phone) a device feature could allow the function at one location but stop it at another. A Feature Code changes the behavior for your extension - and all devices connected to it.