Skip to main content
On an outbound call, Nixflex detects when an answering machine picks up instead of a person. Whether the agent leaves a message — and exactly what it says — is a setting, not a prompt trick. With no message configured, the agent hangs up silently. Detected calls are marked voicemail_detected and billed per second like any other call. Every detected call is marked voicemail_detected and your webhook fires as normal.

​
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 speaks your configured message - or nothing

If a voicemail message is configured for the number the call came from, the agent speaks EXACTLY that message, word for word - it never improvises and never invents callback details. If none is configured, it hangs up without speaking.
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.

​
Leaving a message — the setting

Whether the agent leaves a message comes from a setting, and WHERE that setting lives depends on how the number joined your account. There are two worlds, and they never mix: Numbers imported from the dashboard follow the agent’s setting. Turn on Leave a Voicemail Message under the agent’s Call Actions and type the message — or set it over the API:
Numbers imported over the API use only their own per-number setting — the agent’s message never applies to them. One agent often serves many customers’ numbers, and one customer’s message must never play on another’s line. A number with nothing set hangs up silently on voicemail. Give a number its own message:
The agent speaks the configured text word for word — it never rewrites it, never adds to it, and never invents a callback number.
Enabling without a message is rejected with voicemail_message_required — an agent told to leave a message needs words to say. Turning the toggle off (or clearing the message) returns the number to silent hangup.

​
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:

​
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.

​
Which to use

Detection itself is free either way — the call is billed per second like any other call. 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.

​
On the call record

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