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On an outbound call, Nixflex detects when an answering machine picks up instead of a person. By default the agent leaves a short message and ends the call. You control what it says — or whether it says anything — from your prompt. Every detected call is marked voicemail_detected and your webhook fires as normal.

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How it works

1

The AI reads the first thing it hears

A voicemail greeting mentions leaving a message, a beep or tone, “you have reached”, “not available”, or the equivalent in any language. The AI recognises it from the transcript, not from the audio, so it works in every supported language.
2

It leaves a short message

The agent says who is calling and why, in one or two sentences, taken from your prompt. It waits for the greeting to finish first, so the message lands after the beep.
3

The call ends

No conversation, no waiting for an answer. The call is recorded as voicemail_detected.
Detection is always on, free, and needs no setup.

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Control it from your prompt

The message is built from your prompt. There is no separate voicemail field — anything the agent should say on a machine goes in the prompt like everything else. Leave a message with a callback number:
Hang up without leaving anything:
The [VOICEMAIL] tag is what marks the call as voicemail. Keep it in, or the call is recorded as a silence timeout instead and your campaign counts are wrong. Say nothing about the reason for the call:
The agent builds the voicemail message from your prompt, so anything in your prompt can end up spoken onto a machine that other people in the household can play back. If your call involves health, financial or other private details, say in your prompt what the agent may and may not mention on a voicemail.
If your prompt has no clear purpose, there is nothing to say and the agent ends the call silently.

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If your carrier’s greeting is unusual

Detection works out of the box on normal answering machines in any language — you do not need to configure anything. Some networks word things differently, and some business lines play a switchboard message before the voicemail. If you find calls on a particular network are not being detected, describe what the agent will hear:

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Carrier AMD — faster, but no message

AMD (Answering Machine Detection) is a carrier-level check that listens to the audio and reports human or machine within a few seconds, before the greeting finishes.
AMD ends the call — it never leaves a message. If you turn AMD on, your agent hangs up around 4 seconds in and your voicemail message is never spoken, whatever your prompt says. AMD is for campaigns that want to skip machines cheaply, not for leaving messages.
  • Off by default. Enable it per agent with the Voicemail detection toggle under Call Controls.
  • Works on both carriers. Twilio and Telnyx each provide their own machine detection, and Nixflex treats the result identically.
  • Speed: around 4 seconds, versus around 10 for the built-in detection.
  • Cost: roughly $0.0075 per answered call, billed on your own telephony account.
  • When AMD ends the call, it happens before the agent speaks, so the call has no transcript, sentiment, or summary. This is expected.
If both are active, AMD wins — it is faster, and the call is over before the built-in detection reaches a decision.

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Which to use

Leave AMD off unless you have a reason to want a fast hang-up. The built-in detection is free, works on every carrier and language, and is the only way to leave a message.

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On the call record

Use voicemail_detection_method to tell a free detection from a paid one when reviewing a campaign.