• Code: 421
• Description: Busy Tone is detected after sending NSF Signal.
• Causes: Remote station disconnected the line. Wrong number is dialed.

• Code: 422
• Description: Content of NSF (or DIS) or NSC (or DTC) was invalid.
• Causes: There is an incompatibility.

• Code: 427
• Description: DCN received to NSF/CSI/DIS transmitted
• Causes: The interface is incompatible.

• Code: 433
• Description: .T.30 Protocol abnormal.
• Causes: Defective remote station.

• Code: 434
• Description: CD (response from Modem) did not turn OFF within 180 sec. after receiver detected FLAG signal.
• Causes: Remote unit is defective. SC PCB or MJR PCB are defective.

• Code: 436
• Description: DCN received after transmitting FTT.
• Causes: Transmitter is defective or incompatible. Line quality is poor.

• Code: 456
• Description: Received relay transfer request or confidential document to distribute to an end receiving station or all confidential mailboxes are used.
• Causes:

• Code: 459
• Description: Failed training in Phase C.
• Causes: Line quality is poor. (Training signal is distorted due to line noise) SC PCB or MJR PCB are defective.