Busy override is a function of the private branch exchange that allows the calling party

Calling party

The calling party is a person who initiates a telephone call over the public switched telephone network, usually by dialing a telephone number....

 to override the busy signal

Busy signal

A busy signal in telephony is an audible or visual signal to the calling party that indicates failure to complete the requested connection of that particular telephone call....

 on the called party

Called party

The called party is a person who answers a telephone call. The person who initiates a telephone call is the calling party....

 in order to break into the ongoing conversation.

Before breaking in most PBX announce the incoming call by a distinctive sound

Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 signal

Signal (electrical engineering)

In the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time-varying or spatial-varying quantity....

 or tone

Musical tone

A musical tone is a steady periodic sound. A musical tone is characterized by its duration, pitch, intensity , and timbre . The notes used in music can be more complex than musical tones, as they may include aperiodic aspects, such as attack transients, vibrato, and envelope modulation.A simple...

, but in most cases this can be disabled by software.

The Busy override function was originally intended to allow the called party to receive a high priority or a very important call no matter what.

The same feature is often provided for toll (long distance) operators.