Default Tools

Vapi voice assistants are given additional functions: transferCall,endCall, and dtmf (to dial a keypad with DTMF). These functions can be used to transfer calls, hang up calls, and enter digits on the keypad.

Transfer Call

This function is provided when transferCall is included in the assistant’s list of available tools (see configuration options here). This function can be used to transfer the call to any of the destinations defined in the tool configuration (see details on destination options here).

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{
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  "model": {
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    "provider": "openai",
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    "model": "gpt-3.5-turbo",
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    "messages": [
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      {
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        "role": "system",
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        "content": "You are an assistant at a law firm. When the user asks to be transferred, use the transferCall function."
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      }
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    ],
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    "tools": [
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      {
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          "type": "transferCall",
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          "destinations" : {
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            {
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              "type": "number",
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              "number": "+16054440129"
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            }
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          }
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      }
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    ]
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  }
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}

End Call

This function is provided when endCall is included in the assistant’s list of available tools (see configuration options here). The assistant can use this function to end the call.

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{
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  "model": {
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    "provider": "openai",
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    "model": "gpt-3.5-turbo",
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    "messages": [
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      {
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        "role": "system",
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        "content": "You are an assistant at a law firm. If the user is being mean, use the endCall function."
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      }
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    ],
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    "tools": [
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      {
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          "type": "endCall"
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      }
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    ]
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  }
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}

Send Text

This function is provided when sms is included in the assistant’s list of available tool (see configuration options here). The assistant can use this function to send SMS messages using a configured Twilio account.

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{
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  "model": {
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    "provider": "openai",
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    "model": "gpt-3.5-turbo",
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    "messages": [
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      {
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        "role": "system",
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        "content": "You are an assistant. When the user asks you to send a text message, use the sms function."
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      }
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    ],
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    "tools": [
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      {
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        "type": "sms",
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        "metadata": {
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          "from": "+15551234567" 
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        }
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      }
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    ]
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  }
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}

Dial Keypad (DTMF)

This function is provided when dtmf is included in the assistant’s list of available tools (see configuration options here). The assistant will be able to enter digits on the keypad.

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{
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  "model": {
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    "provider": "openai",
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    "model": "gpt-3.5-turbo",
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    "messages": [
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      {
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        "role": "system",
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        "content": "You are an assistant at a law firm. When you hit a menu, use the dtmf function to enter the digits."
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      }
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    ],
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    "tools": [
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      {
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          "type": "dtmf"
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      }
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    ]
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  }
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}

There are three methods for sending DTMF in a phone call:

1. In-band: tones are transmitted as part of the regular audio stream. This is the simplest method, but it can suffer from quality issues if the audio stream is compressed or degraded.
2. Out-of-band via RFC 2833: tones are transmitted separately from the audio stream, within RTP (Real-Time Protocol) packets. It’s typically more reliable than in-band DTMF, particularly for VoIP applications where the audio stream might be compressed. RFC 2833 is the standard that initially defined this method. It is now replaced by RFC 4733 but this method is still referred by RFC 2833.
3. Out-of-band via SIP INFO messages: tones are sent as separate SIP INFO messages. While this can be more reliable than in-band DTMF, it’s not as widely supported as the RFC 2833 method.

Tool Effectiveness

To evaluate this tool, we set up a Vapi assistant with the DTMF tool enabled and conducted calls to a range of IVR systems, including a Twilio IVR (configured via Studio Flows) and several third-party IVRs such as pharmacies and insurance companies.

Testing Methodology

We called and navigated through the IVRs using three different strategies:

1. Direct Dialpad: calling from a personal phone and dialing options using the dialpad.
2. Vapi DTMF Tool: an assistant configured with the DTMF tool.
3. Manual DTMF Sound: calling from a personal phone and playing DTMF tones generated by software. (similar approach as the Vapi DTMF Tool)

Key Findings

• The assistant successfully navigated some of the third-party IVRs.
• The assistant encountered issues with Twilio IVRs, likely due to Twilio’s preference for RFC 2833.
• Observed occasional delays in DTMF tone transmission, which may affect effectiveness with IVRs that have short timeouts.

Conclusion

The tool’s effectiveness depends on the IVR system’s configuration and DTMF capturing method. We are working to improve compatibility and reduce transmission delays for broader and more reliable support.

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{
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  "functions": [
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    {
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      "name": "bookAppointment",
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      "description": "Used to book the appointment.",
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      "parameters": {
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        "type": "object",
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        "properties": {
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          "datetime": {
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            "type": "string",
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            "description": "The date and time of the appointment in ISO format."
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          }
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        }
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      }
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    }
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  ]
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}