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Ultravox provides comprehensive support for DTMF (Dual-Tone Multi-Frequency) tones, enabling both sending and receiving tones during phone calls. This enables AI agents to interact with traditional phone systems and allows you to build voice applications that can respond to keypad inputs.
DTMF and WebRTC
Due to the audio codec used in WebRTC connections, DTMF tones are inaudible when using WebRTC. The playDtmfSounds tool is intended for use with telephony integrations.

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Receiving DTMF Tones

Ultravox automatically converts incoming DTMF tones to text, making it easy to build interactive voice applications that respond to keypad input. When a caller presses keys on their phone keypad, the tones are converted to text that your AI agent can understand and respond to. For example, if a caller presses “5” on their keypad, your agent will receive this as text and can respond accordingly: 
// Example system prompt for an agent that handles DTMF input
{
  "systemPrompt": `You are an automated phone system.
    When a caller joins, say: "Welcome! Press 1 for sales, 2 for support, or 3 for billing."
    If they press 1, transfer them to sales using the transfer tool.
    If they press 2, transfer them to support.
    If they press 3, transfer them to billing.
    If they press any other key, ask them to try again with a valid option."`
}

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Sending DTMF Tones

The built-in playDtmfSounds tool allows your AI agent to send DTMF tones, which is useful for navigating Interactive Voice Response (IVR) systems or other phone trees. To enable the tool, add it to the selectedTools array when creating a call or call stage: 
// Example request body for creating a call with DTMF capability
{
  "systemPrompt": "You are a helpful assistant. When prompted to dial an extension, use the 'playDtmfSounds' tool to send the appropriate tones.",
  "selectedTools": [
    { "toolName": "playDtmfSounds" }
  ]
}
The playDtmfSounds tool accepts a string parameter named digits and works with the following tones: 0-9, *, #, A-D. For example: 
// Example of using the playDtmfSounds tool to dial an extension
{
  "digits": "123#"  // Will play tones for 1, 2, 3, and # in sequence
}
In-band vs out-of-band DTMFThe playDtmfSounds tool is so named because it defaults to playing DTMF tones as audio (in-band) for compatibility with all phone systems.However, out-of-band DTMF (where tones are sent as signals rather than audio) is generally preferable for its improved reliability and may be required by some telephony stacks. The same tool will use out-of-band DTMF when the call’s medium supports it and Ultravox has the information available to support it. To use out-of-band DTMF with supported telephony platforms such as Telnyx or Plivo, add your telephony credentials so Ultravox can send out-of-band DTMF on your behalf where supported by your provider. (Alternatively you can create your own tool of course.)

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Common Use Cases

  • Building interactive phone trees or IVR systems
  • Creating agents that can navigate existing phone systems
  • Enabling quick responses through keypad input
  • Collecting numeric input (e.g., account numbers, PIN codes)
  • Building hybrid voice/keypad interfaces