Do Not Disturb is a common feature in enterprise telecommunications. When it is enabled, it prevents the telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 from ringing. The implementation of this feature is vendor-specific. In some cases, the ringer just does not ring and the called party is thus not alerted. Usually, though, the phone just acts as being busy. Depending on the infrastructure, the caller may end up on the called party's voice mailbox, sometimes after a certain delay.

Some special configurations combine DND with other features like call forwarding

Call forwarding

Call forwarding , in telephony, is a feature on some telephone networks allowing an incoming call to a called party to be redirected to a third party. For example, the third party may be a mobile telephone, voicemail box or other telephone number where the desired called party is situated. It was...

, where calls to a party with DND on are forwarded to another phone.