What DTMF types are supported

DTMF (Dual-Tone Multi-Frequency) - these are signals generated when pressing keys on a phone that are used to transmit digital data, such as numbers or commands, through telephone networks.

We support the following DTMF transmission types:

1. RFC 2833 (default for all clients)

   This protocol uses RTP packets with Named Telephony Event (NTE) payload type for transmitting DTMF signals. Unlike the In-band method, where the audio tone is audible, with RFC 2833, DTMF signals are transmitted as separate packets, and no tones are heard in the voice stream. Instead, NTE packets contain information about the DTMF signal. For example, if a client presses the "1" key, the "Event ID" field will transmit the value DTMF One 1 (1). This means that the other party won't hear a tone when pressing the key, but the press information will be correctly transmitted and interpreted by the system.

2. In-band

   With this method, DTMF signals are sent directly in the RTP voice stream. This method works well with the G.711a codec since the voice stream is not compressed, and DTMF signals are transmitted along with voice. However, if compression is used, for example with the G.729 codec, problems arise. When compressing the voice stream, sound quality deteriorates, and the DTMF tone becomes distorted, which can lead to incorrect signal perception. Although compression doesn't significantly affect voice perception, compressed DTMF signals lose clarity and may be misinterpreted. Therefore, the In-band method is only possible when using the G.711a codec, which transmits the signal without quality loss.

3. IFO (Information Only)

   This method involves transmitting DTMF signals as separate information, but unlike RFC 2833, the information is transmitted through text or control packets. It is mainly used in specialized networks and applications where DTMF signals don't require transmission through voice channels.

4. Auto

   This method automatically selects the most suitable way to transmit DTMF depending on network conditions and configuration, applying preferences to RFC 2833 or In-band, depending on which method will be more effective for the specific situation.

Thus, to ensure accuracy and reliability of DTMF signal transmission, it is recommended to use RFC 2833, especially when using compression codecs such as G.729, where the In-band method may lead to data loss.