DTMF Dual-Tone Multi-Frequency

Which DTMF method does sipcall recommend?

We recommend transmitting DTMF using RFC4733 or its predecessor RFC2833. This method offers the highest compatibility.

• In cases where compressed audio codecs (e.g. G.722) are used,
  the above-mentioned RFC method must be used for transmitting DTMF tones.
• Some phone systems, such as Microsoft Teams Telephony, do not support In-Band or SIP-Info.

What is DTMF?

DTMF (Dual-Tone Multi-Frequency) is a tone dialing method used to transmit digits,
symbols, and control commands over telephone lines.
It is used in analog and digital telephone networks as well as in VoIP systems.

Areas of Application

• Telephone menu control (e.g. IVR - Interactive Voice Response)
• Remote control of systems (e.g. call forwarding, voicemail control, see Telephone Service Codes)
• Access control (e.g. door opening via telephone)

Which DTMF methods are supported by sipcall?

RFC 2833 and RFC 4733

• DTMF signals are sent as separate RTP packets.
• Reliable and preferred in modern VoIP environments.
• RFC 2833: older standard
• RFC 4733: newer standard that supports HD codecs

In-Band

• DTMF tones are sent as audio signals directly within the voice stream.
• Prone to distortion due to audio compression.

SIP INFO

• DTMF signals are sent as SIP messages (INFO methods).
• Used less frequently, as it requires an additional signaling layer.