Hi Narin,

Your approach for detecting call response is great, but workable only when the called number is pre-running some application with the code, like continuously sending a DTMF character.


As a standard:
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To detect call response from any number, you have to monitor the ring tone. Just measure the tone length and the silence length between two tones while ring is going after dialing. If the call is accepted then either the silnece will be longer or the tone will be shorter than normal. Just judge on either case.

I have successfully done this. Just omit first tone as at times it is never full length which gives error.

For your DTMF modem:
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In my opinion your approach should work without a problem.

I don't know whether you are making [ Mod ---> Dem ] (one way) communication or [ Modem ---> Modem ] (two way) communication.

Please confirm on this and then its only a question of designing a good protocol.

In your next mail try to explain your application with detail of hardware.

Let's not give up.

Simon


PS: by the way why don't you try a standard FSK modem ??