Oki OKIFAX 5750 Service Manual page 304

2.9.2.4 TEL/FAX Automatic Switching
This function is used for the purpose of TEL/FAX automatic switching as follows.
(1) If the machine detects a call with a CNG signal indicating an auto send facsimile call, it starts an automatic document receiving operation.
(2) If machine detects a call without a CNG signal, machine generates the buzzer sounds as a telephone call. The calling person can hear a "ring back" tone
within a predetermined time.
If the operator at the called side does not lift the handset within the predetermined time, the machine automatically starts a document receiving operation.
Voice conversation will automatically be available through the internal handset by lifting up the handset while the call buzzer is sounding.
Note:
1: The predetermined time is selectable between 20 or 35 sec. (Function program No. 10)
2: No ringing signal is sent to the external telephone handset.
3: Choice of message sending level. The level is selectable from 0 to 15 dB in one dB step. (Technical function No. 22)
4: TEL/FAX mode is available by Technical Function No. 08.
Service Guide OKIFAX 5750/5950
Chapter 2 Installation
Page: 111