Oki OKIFAX 5950 Series Maintenance Manual page 350

Test other fax machine under the same
conditions, especially concerning Step 1-4.
Do they transmit normally?
The hardware of the troubled fax machine
may be damaged.
Go to 7.13 (High-speed Modem Test)
to localize the cause.
Description: Protective tone is 1700 Hz/200 ms.
This signal is added to training signal to protect the training signal against echo
as follows.
OF5750/5950 Series January 2001
A
No
Yes
TX
TR
message signal
Protective tone
1700 Hz/200 ms
7 - 9
Check the line and the network.