How does an AI voice agent know when the caller has finished speaking?
An AI voice agent decides the caller has finished speaking in two steps: voice activity detection notices when speech stops, and endpointing judges whether that silence is the end of the turn or just a pause. Better systems also look at the words themselves, so 'my budget is around' is treated as unfinished even if the caller stops for a moment.
Why this is harder than it sounds
People pause constantly mid-sentence: to think, to recall a number or because someone in the room distracted them. If the agent replies at every pause, it interrupts. If it waits too long, it leaves awkward silences and feels slow. Getting this balance right is one of the main differences between a voice agent that feels natural and one that does not.
Step one: voice activity detection
Voice activity detection, or VAD, is a small, fast model that checks short frames of audio many times a second and labels each as speech or not speech. It handles timing rather than meaning. A good VAD ignores background hum and brief noises, and reacts within tens of milliseconds when speech starts or stops.
Step two: endpointing
Endpointing is the decision layer on top. The simplest approach is a fixed silence timeout, say 700 milliseconds, after which the turn is considered over. That works for short answers like 'haan' but cuts people off when they pause mid-thought.
More advanced endpointing combines silence with the content of the transcript. Sentences that are grammatically incomplete, end on a filler like 'umm', or trail off on 'aur' or 'and' get more time. Complete answers to a direct question, such as a yes or a date, get a quicker response.
Signals a smart endpointer uses
- —Length of the silence since the last word.
- —Whether the transcript forms a complete thought.
- —Whether the agent just asked a question that expects a short answer.
- —Falling pitch at the end of a sentence, which often signals completion.
- —Filler sounds and connectors that suggest more is coming.
What it means for callers
Well-tuned endpointing is invisible; callers only notice it when it fails. When evaluating any voice agent, including Outpero, try giving a phone number in two halves with a pause in the middle, and see whether the agent waits for the full answer or jumps in after the first five digits.
Want to hear an AI voice agent handle this on a real call?
Follow-up questions
Is endpointing the same as barge-in?
+
No. Endpointing decides when the caller has finished so the agent can start talking. Barge-in handles the opposite case, when the caller starts talking while the agent is still speaking and the agent has to stop. Both rely on voice activity detection, and together they make up what is usually called turn-taking.