Answering machine detection (AMD)
Answering machine detection (AMD) is the process of classifying whether an answered outbound call reached a live person or an automated greeting, such as voicemail or an answering machine.
The distinction is harder than it sounds because both people and machines begin with speech. An AMD system may examine the timing, duration, pauses, tone, and structure of the opening audio. A short greeting followed by silence may indicate a person waiting for a response, while a longer uninterrupted greeting and a recording cue may indicate voicemail. Some systems make an early decision; others wait for more evidence.
Every decision involves a tradeoff between speed and confidence. Waiting longer can improve classification, but a real person may hear silence before the agent begins speaking. Deciding too quickly can mistake a long human greeting for voicemail or treat an automated greeting as a person. Background noise, business switchboards, custom greetings, language differences, and carrier announcements add uncertainty.
For an AI phone agent, AMD determines which call flow should run after an outbound call connects. A person can enter the live conversation. A detected machine can trigger a hangup, a configured voicemail message, or another campaign outcome. The classification should also be stored as a disposition so teams can separate genuine conversations from machine answers when reviewing results.
AMD is related to voicemail detection but is not always identical. AMD usually describes the broader live-person-versus-machine classification at answer time. Voicemail detection may focus specifically on recognizing a mailbox and deciding whether or when to leave a message. Product interfaces sometimes use the terms interchangeably, so buyers should verify the actual behavior.
Teams should treat AMD as probabilistic rather than perfect. Test it against the caller population, greeting styles, languages, and destinations the campaign will use. Review both kinds of error: messages incorrectly played to people, and live answers incorrectly treated as machines. The desired balance depends on the call's purpose and how costly a delayed greeting or wrong disposition would be.
In practice on ThunderPhone
ThunderPhone's voicemail policy can let the agent prompt decide, hang up, or leave a configured message when voicemail is detected. Campaigns can retry voicemail outcomes according to their retry policy.