Dual-Tone Multi-Frequency

DTMF (Dual-Tone Multi-Frequency) refers to the signaling tones generated when a caller presses keys on a telephone keypad. Each key produces a unique combination of two audio frequencies that the system can detect and interpret. In VICIdial, DTMF is used for IVR navigation, call menu selections, and agent-initiated transfer operations.

How It Works in VICIdial

VICIdial and Asterisk support three DTMF transmission methods: RFC 2833 (out-of-band in RTP packets), SIP INFO (signaling-based), and in-band audio detection. The method used depends on the SIP trunk configuration and the capabilities of the endpoints involved. RFC 2833 is the most reliable and is the recommended default for VICIdial carrier trunks.

When a caller reaches a VICIdial call menu, the system listens for DTMF input and routes the call based on the pressed digit — for example, “Press 1 for Sales, Press 2 for Support.” DTMF detection is also used during AMD processing, where some answering machine greetings can be bypassed by sending a DTMF tone.

Agents use DTMF when performing warm or blind transfers, dialing extension numbers, or navigating external IVR systems during three-way calls.

Why It Matters

Reliable DTMF detection is essential for any VICIdial deployment using inbound IVRs or automated call routing. DTMF failures — where key presses are not recognized — cause caller frustration, misrouted calls, and abandoned interactions. The most common cause is a DTMF mode mismatch between VICIdial and the carrier, where one side sends RFC 2833 but the other expects in-band audio.

When troubleshooting DTMF issues, verify that both the carrier trunk and the agent endpoint are configured to use the same DTMF method. RFC 2833 should be the first choice for all SIP and PJSIP connections.