Sharp FO-4450 - Manual (Page 130)

Messages and Signals
128
Audible signals
Continuous tone 3 seconds Indicates the end of transmission,
reception, or copying.
Intermittent tone
(3 beeps) 5 seconds
(1 second on, 1
second off) Indicates incomplete transmission,
reception, or copying.
Rapid intermittent
tone 35 seconds
(0.7 seconds on,
0.3 seconds off) Indicates that an extension phone
connected to the fax is off hook.