VAPI Voicemail Detection Setup Checklist

If you’re building AI calling flows with VAPI, you already know:

Voicemail detection is the silent killer of good call performance.

If your AI agent talks over a voicemail greeting, burns minutes, or leaves awkward half-messages, you waste money and create a poor customer experience.

The good news? You can fix it. Here's a plain-English, step-by-step setup to properly detect and handle voicemails with VAPI — and avoid common traps.

Choose the Right Voicemail Detection Mode

VAPI now primarily uses two voicemail detection methods:

LLM-Based Detection with Function Calling (Recommended)
Experimental Gemini Flash Detection (Optional, less reliable)

The best practice today is to configure your agent to allow the LLM (such as GPT-4o) to monitor transcribed audio in real-time. When it detects a voicemail greeting, it triggers a dedicated Voicemail function to leave a prepared message.

Where to set it:

In your Assistant Settings, configure the function calling schema
Make sure Voicemail is registered as an available function

See VAPI Docs on Handling Calls

Note: Twilio's AMD (Answering Machine Detection) is now considered a legacy fallback. It is available but generally not recommended for new setups.

Prepare a Dynamic Voicemail Message Before the Call Starts

One of the biggest upgrades in modern AI calling flows is pre-generating the voicemail.

Instead of reacting on the fly, set up a relevant, high-quality voicemail message at call initiation.

Example workflow:

Pull prospect info (name, company, lead source)
Generate a voicemail script dynamically based on the purpose of the call
Pass that voicemail into the agent's function calling options

Example dynamic voicemail:

"Hi Sarah, this is Jamie from Modern HVAC. I saw you requested a quote for your new system. Give us a call back at (555) 123-4567 or visit modernhvac.com to get scheduled. Thanks so much!"

Setting this up gives your agent a mission-specific fallback if voicemail is detected.

Configure Max Detection Retries Carefully

Setting retry counts incorrectly can cause your agent to lag or error out.

Recommended default: set Max Detection Retries to 3.

This ensures a balance: enough attempts to detect voicemail, without unnecessary processing delays.

Where to adjust:

In your Call Settings configuration for the agent

Tip: Excessive retries were a known cause of publishing failures in early versions of VAPI. Stay conservative unless advised otherwise.

Plan a Fallback for No-Answer Calls

Even when voicemail detection is reliable, not every call will result in a pickup or a clear voicemail opportunity.

Strong fallback options:

Trigger an automated SMS follow-up ("Sorry we missed you! Text us back to reschedule.")
Log call outcomes automatically in Airtable, Google Sheets, or CRM
Queue a callback task for your team

Configure these actions through:

VAPI Assistant "Call Ended" hooks
Connected workflows via Make.com or Zapier

Learn More About Call Handling

Bonus: SMS follow-ups often yield higher re-engagement than a single voicemail alone.

Test Real-World Scenarios (Not Just Simulations)

Before going live, you need to test:

Outbound call answered by a live person
Outbound call that reaches voicemail
Busy signal or immediate call failure

Checklist:

Does your agent correctly detect voicemail via the LLM function call?
Does it deliver the dynamic voicemail you prepared?
Does it trigger fallback actions properly when no answer is detected?

Testing Tip: Always use real devices (not just simulators) to verify call behavior across carriers.

Common Pitfalls to Watch For

Speaking too soon: Leave a slight pause before initiating dialogue. Let the system process the greeting.
Relying on Twilio AMD: It is outdated compared to LLM detection. Avoid unless necessary.
Skipping fallback setups: Calls can dead-end without a fallback plan, wasting valuable leads.
Overconfiguring detection retries: It slows the system and risks validation errors.

Voicemail detection is no longer a "nice-to-have" — it is critical infrastructure if you want your AI receptionists to sound natural and efficient.

Setting up proper LLM-based voicemail detection, preparing dynamic messages ahead of time, and planning reliable fallbacks ensures your calls perform the way real customers expect.

If you're serious about winning in AI-driven communication, get this part right early.

Need deeper help? Explore VAPI's latest Call Handling Documentation or join their community forums to learn from real-world builder experiences.

Another Way: Let AgentVoice Handle It

We’ve engineered a better approach. AgentVoice dynamically generates voicemail messages before each call starts, using the prospect’s details and the user's campaign goals. If a voicemail is detected, our platform automatically delivers a customized, natural message without wasting time or calls. If you’d like to skip the trail and error, give AgentVoice a try.

AgentVoice Dashboard.