Voice Mail Detection

Voicemail is basically a digital answering machine. When you can’t pick up, callers can leave a message so you don’t miss anything important. It’s especially handy if you’re in a meeting, driving, or just can’t get to the phone in time.

The Main Problem

If a lot of your calls are landing in voicemail, you could be spending too much time and money on calls that never connect to a real person. This leads to wasted resources, and sometimes missed business opportunities.

The Solution: Early Voicemail Detection

By detecting voicemail right away, your VAPI Assistant can either hang up (if leaving a message isn’t necessary) or smoothly play a recorded message. This cuts down on useless call time and makes your entire call flow more efficient.

Two Ways to Detect Voicemail

1. Using Twilio’s Voicemail Detection

Twilio has built-in features to detect when a machine picks up. You configure these settings in your VAPI Assistant so it knows when a voicemail system has answered instead of a live person.

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voicemailDetection: {
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  provider: "twilio",
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  voicemailDetectionTypes: [
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    "machine_start",
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    "machine_end_beep",
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    "machine_end_silence",
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    "unknown",
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    "machine_end_other"
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  ],
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  enabled: true,
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  machineDetectionTimeout: 15,
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  machineDetectionSpeechThreshold: 2500,
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  machineDetectionSpeechEndThreshold: 2050,
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  machineDetectionSilenceTimeout: 2000
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}

• provider: Tells VAPI to use Twilio’s system.
• voicemailDetectionTypes: Defines the events that mean “voicemail.”
• machineDetectionTimeout: How many seconds to wait to confirm a machine.
• The other settings let you fine-tune how quickly or accurately Twilio identifies a machine based on speech or silence.

Quick Reference

2. Using VAPI’s Built-In Voicemail Tool

VAPI also has an LLM-powered tool that listens for typical voicemail greetings or prompts in the call’s audio transcription. If you prefer an approach that relies more on phrasing and context clues, this is a great option.

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{
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  ...yourExistingSettings,
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  "model": {
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    "tools": [{ type: "voicemail" }]
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  }
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}

Here, tools: [{ type: "voicemail" }] signals that your VAPI Assistant should look for keywords or patterns indicating a voicemail greeting.

Combining Both Approaches

For the best of both worlds, you can enable Twilio’s detection and the built-in voicemail tool at the same time:

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{
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  ...yourExistingSettings,
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  voicemailDetection: {
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    provider: "twilio",
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    voicemailDetectionTypes: [
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      "machine_start",
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      "machine_end_beep",
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      "unknown"
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    ],
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    enabled: true,
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    machineDetectionTimeout: 15
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  },
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  model: {
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    tools: [{ type: "voicemail" }]
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  }
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}

When one method doesn’t catch it, the other might—boosting your overall detection accuracy.

Tips for Better Voicemail Handling

1. Adjust Detection Timing

   Lower machineDetectionTimeout (e.g., to 5 seconds) if you want the system to decide faster. But remember, shorter timeouts can lead to occasional false positives.

2. Fine-Tune Speech and Silence Thresholds

   For example:

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   {
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     "provider": "twilio",
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     "enabled": true,
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     "machineDetectionTimeout": 5,
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     "machineDetectionSpeechThreshold": 2400,
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     "machineDetectionSpeechEndThreshold": 1000,
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     "machineDetectionSilenceTimeout": 3000
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   }

   These values tweak how quickly Twilio “listens” for human speech or background silence.

3. Think Through Your Call Flow

   • Give It Time: If you’re leaving a message, you might want to increase startSpeakingPlan.waitSeconds so the detection has enough time before the tone.
   • firstMessageMode: Setting it to assistant-waits-for-user can also give you smoother call handling—your assistant won’t barge in if someone unexpectedly picks up late

What Happens When a Call Ends?

• Detected Voicemail + No Message: The call will end, and you’ll see a reason like customer-did-not-answer.
• Detected Voicemail + Have a Message: Your assistant leaves the recorded message, and the call ends with a reason like voicemail.

Testing and Next Steps

1. Make a Test Call: Dial a known voicemail number and watch how quickly (and accurately) your VAPI Assistant identifies the machine.
2. Tweak Settings: Adjust your timeout and threshold values based on real-world performance.
3. Repeat: Keep testing until you’re confident your configuration is catching voicemail reliably without cutting off real people.

By following these steps, you’ll save time, improve call-handling efficiency, and ensure your system feels more professional. If you need to fine-tune or add new features later, you can always revisit these settings and make quick adjustments.