SignalUpperFrequency and SignalLowerFrequency define a range of frequencies on the line. Sound within these bounds is recognized by VoiceAngel as signal sound ( ring back tone or busy tone). Any sound with frequency beyond these bounds will be taken as non-signal sound.

When VoiceAngel "listens" to the line, patterns of signal and non-signal cause different events and behaviours.

For example, if non-signal sound is detected after signal sound, VoiceAngel will think that someone has picked up the phone and said something. (Obviously, it cannot interpret or understand speech, other than to know that it is "non-signal"). In this case, an CALL_STATUS_CONNECTED message will be invoked in CallStatus event. But this is not always accurate. If the called party was to drop his mobile telephone just after you dialled, a voice prompt will be heard, and voiceangel can not recognize if the voice is from a human or a machine, so the Connected event is still raised.

Sometimes non-signal sound is heard but the line has not been connected successfully. For example ,if the callee has shut down his mobile telephone or he is busy in a call process, VoiceAngel may hear a voice prompt directly without any signal tone. In this case a CALL_STATUS_VOICE_PROMPT_DETECTED message will be invoked in CallStatus event.

Setting DownFrequency at design time, makes it apply to all lines. Setting it at run time means it will only apply to the current line.

See also: Judging Telephony Line Status