The process of establishing a call differs depending upon whether the application is acting as a calling fax terminal or as a called fax terminal. Use one or more call control functions from the NCC service to establish a call. Refer to the Natural Call Control Service Developer's Reference Manual for information on call control.
Call progress analysis in the NCC service includes the capability to detect CED tones. A CED tone is a three second 2100 Hz tone indicating that a fax terminal has answered.
Enable CED tone detection when invoking the nccPlaceCall NCC service function. The application generates an NCC service call progress event if it detects a CED tone. If no CED tone is detected, the application provides a way to handle a call answered by a person or by a modem rather than by a fax machine.
The application invokes the nccAnswerCall NCC service function and the adiStartToneDetector ADI service function to answer an inbound call and detect CNG tones. The calling fax terminal sends a CNG tone. A CNG tone is a 0.5 second 1100 Hz tone that indicates a fax terminal is calling.
If an application needs to handle both fax and human callers, it should start a special tone detector to detect CNG tones in conversation state. The application starts a fax session only if a CNG tone is detected. In a fax-only application, a fax session may be started as soon as the call is answered.