What is the purpose of using DTMF in sip based softphone?

Question

DTMF is used in some sip based softphone to handle payload type. But the process and the importance of using DTMF is not clear to me. Open source softphone like Red5phone and Sipdroid uses DTMF. Would anyone please explain the issue or point me somewhere I can find good enough explanation.

Thanks.

Answer 1

If you are calling to some machine-answered phone, it may asks you to choose from multiple menu items, then you will press a number for your choice. This number should be transmitted using DTMF to peer.

This is what DTMF matters.

I hope I didn't misunderstand your question.

For the RTP payload of DTMF, please reference RFC2833.

Answer 2

The question is a but vague, so without going into details DTMF requires special handling in SIP because the low bandwidth, speech-oriented codecs do not transfer it well. As long as you stick with G711, you don't need to care, just generate the tones in-band. (as part of the voice stream.)

To use more advanced codecs, you'll have to support transferring the dialpad keypresses as RTP events or SIP INFO messages (preferably both to be able to negotiate.)

http://www.voip-info.org/wiki/view/SIP+DTMF+Signalling

Answer 3

DTMF is short for Dual-tone multi-frequency signaling and it is used for telecommunication signaling over telephone lines. DTMF is applied in the voice-frequency band between telephone handsets and other communications devices and the switching center. It allows tone dialing in push-button telephones. (For more information you can visit the following page: voip-sip-sdk.com/page234)

Answer 4

I can correlate the use and importance of DTMF to IVR ( Interactive Voice Respondent ) Menus, as in my project .

Menus such as Failed call handling is usually provided with a IVR menu saying . press 1 to send a voicemail . press 2 to call back . press 3 to do something else ....

There is usually a drop down number-pad present to make the selection .While sip softphone are used on desktop keyboard can be used to enter the number .For phones , tablets or Ip phones the number pad existing or in case of touch phones the appearance of number pad is provided .