BEEP! is the noise consoles make when a mission has been received for a response center. These beeps are loud, and high pitched, and grate on the hearing of most agents.
Beeps appear in most mission logs as as a [BEEP!] of varying length, likely depending on how the console itself is feeling and what the agents themselves are doing at the time.
Occasionally, the console might be sufficiently threatened into making a muted [Bip.] Evidence that some consoles might have some kind of vague sentience comes from the fact that, if ignored, they have been known to add profanities to their alert noises, viz: [BEEEEEEEPBLOODYBUGGERINGBEEEEEEEEP!]
Some agents reprogram their consoles for something less grating. One console plays "Ride of the Valkyries," and another plays "Fireplace Theme" (an agitato bonus track in the Myst soundtrack).
In a TARDIS, the Cloister Bell announces missions.