An all call is a general term for a broadcast message. In particular, it may be used to refer to an automated phone notification system that will quickly call individuals with a recorded message in the event of a weather-related school closing or emergency situation.
An example of an ALL-CALL in a school would be, "Please excuse the interruption, please report to the Auditorium at this time. Thank you."
On a Norstar system, you can make an "ALL-CALL" by entering Feature 630. This will broadcast a message throughout the entire campus.
On Panasonic systems, you can make an "ALL-CALL" by entering the numbers 339. This activates the entire campus' paging devices.
An "ALL-CALL" function may also be referred to as a "PAGE".
References
All call Wikipedia(Text) CC BY-SA