docs(agents): document the echo guard setting

agents/build/configuration.mdx

@@ -59,8 +59,24 @@
   - `low` (**Hard to interrupt** in the Builder): The agent stops only for a firm, worded interjection, talking through background noise and short acknowledgements.
   - `balanced` (default): Interrupts on normal speech.
   - `high` (**Easy to interrupt**): The agent stops on the caller's first word.
 - `conversation.interruption_ignore_phrases` (up to 50 entries of 40 characters): Backchannels such as `"uh-huh"` or `"okay"` that never interrupt the agent when the caller says only them. Matching ignores case and punctuation. Send `[]` to clear the list.
 
+### Echo guard
+
+Callers' devices and the phone network normally cancel the agent's echo, but in a few calls (under 3%) some echo gets through in the agent's first seconds and cuts it off.
+
+`conversation.echo_guard` (**Echo guard** in the Builder) prevents this. For 3 seconds after the agent first starts talking, the caller can't interrupt it, and what the caller says over the agent is dropped. [Keypad entries](/agents/telephony/keypad#keypad-input) and the [web SDK](/agents/deploy/web-sdk)'s `interrupt()` still work.
+
+Each [channel](/agents/build/dynamic-variables#system-variables) has its own key. Set it to `true`, `false`, or `null` for the default.
+
+| Key | Builder label | Default |
+|---|---|---|
+| `web_voice` | **Web calls**, including [Preview calls](/agents/test/preview-calls) | On |
+| `phone_inbound` | **Inbound phone calls** | On |
+| `phone_outbound` | **Outbound phone calls** | Off |
+
+Turn it off if callers should be able to interrupt the agent within those 3 seconds.
+
 ### Call duration
 
 `conversation.max_duration_seconds` (60–3600, default `1800`) ends the session automatically when the limit is reached.
@@ -77,7 +93,7 @@
 
 `conversation.timezone` is the default IANA timezone (like `Asia/Tokyo`) the agent uses for dates and times in conversation. Leave it empty for **automatic**: each session follows the caller's device or phone number, falling back to UTC. Set one when your agent serves a single region regardless of who calls. A per-session `timezone` on the [session request](/agents/build/time-timezone) overrides this. See [Time & timezone](/agents/build/time-timezone) for the full resolution order.
 
 ## Autosave and publishing
 
 There is no Save button. Each change is written to the agent's draft moments after you stop editing, and the **Saving… / Saved** indicator at the bottom-left of the page shows the current state. If a save fails, the Builder tells you and keeps your pending edits so nothing is lost.
 
@@ -106,7 +122,7 @@
     "voice_id": "802e3bc2b27e49c2995d23ef70e6ac89",
     "speaking_language": "en"
   },
   "asr": {
     "model": "deepgram:nova-3",
     "multilingual": false,
     "strict_language": false,
@@ -120,6 +136,7 @@
     "interruptible": true,
     "interruption_sensitivity": "balanced",
     "interruption_ignore_phrases": [],
+    "echo_guard": null,
     "speculative_response": true,
     "timezone": ""
   }
@@ -150,7 +167,7 @@
 |---|---|---|
 | `prompt` | System prompt and first-message settings | This page |
 | `voice` | Voice profile, text-to-speech model, speaking language, and expressive mode | [Voice & language](/agents/build/voice-language) |
 | `asr` | Speech recognition model, multilingual recognition, strict language, and recognition keyterms | [Voice & language](/agents/build/voice-language#speech-recognition-model) |
 | `conversation` | Call duration, turn-taking, interruption behavior, timezone, and storage | This page; storage in [Conversation history](/agents/monitor/conversation-history#what-gets-stored) |
 | `tools` | Attached webhook tools and system tool switches | [Tools](/agents/build/tools) |
 | `knowledge_base` | Attached knowledge sources | [Knowledge base](/agents/build/knowledge-base) |