Toshiba CIX series User Training Reference page 5

Commonly used flexible button features.
ABR Auto Busy Redial. Once encountering a busy signal, press this
ACB Auto Call Back.
key to enable the system to redial the busy destination at regular
intervals. The manual code to set is CNF/TRN#441. (to cancel is
#442)
system, press this key to have the system monitor that stations
busy status. Once that station returns to an idle state, your station
is notified and the internal call is connected. The manual code to
set this feature is the number 4. (to cancel #431)
Once encountering a busy station in your