diff --git a/.changeset/quiet-clouds-warm.md b/.changeset/quiet-clouds-warm.md
new file mode 100644
index 000000000..72ef58bcc
--- /dev/null
+++ b/.changeset/quiet-clouds-warm.md
@@ -0,0 +1,5 @@
+---
+'@livekit/agents': patch
+---
+
+Disable the default AEC warmup for outbound SIP calls while preserving explicit settings.
diff --git a/agents/src/voice/agent_session.test.ts b/agents/src/voice/agent_session.test.ts
index ed7fb8452..3dfdf4f82 100644
--- a/agents/src/voice/agent_session.test.ts
+++ b/agents/src/voice/agent_session.test.ts
@@ -1,6 +1,7 @@
 // SPDX-FileCopyrightText: 2026 LiveKit, Inc.
 //
 // SPDX-License-Identifier: Apache-2.0
+import { ParticipantKind, type RemoteParticipant } from '@livekit/rtc-node';
 import { describe, expect, it, vi } from 'vitest';
 import { AgentSession, resolveRecordingOptions } from './agent_session.js';
 import { AgentSessionEventTypes, createUserInputTranscribedEvent } from './events.js';
@@ -8,10 +9,64 @@ import { SpeechHandle } from './speech_handle.js';
 
 type AgentSessionInternals = AgentSession & {
   _agentState: string;
+  _aecWarmupTimer: NodeJS.Timeout | null;
   _userState: string;
   _setUserAwayTimer: () => void;
 };
 
+describe('AgentSession AEC warmup', () => {
+  it.each([
+    [ParticipantKind.SIP, {}, null],
+    [ParticipantKind.SIP, { 'sip.ruleID': 'SDR_inbound' }, 3000],
+    [ParticipantKind.STANDARD, {}, 3000],
+  ] as const)(
+    'uses the call type default for participant kind %s with attributes %o',
+    (kind, attributes, expectedDuration) => {
+      const session = new AgentSession({ vad: null });
+      const participant = { info: { kind }, attributes } as RemoteParticipant;
+
+      session._onRoomIOParticipantLinked(participant);
+
+      expect(session.sessionOptions.aecWarmupDuration).toBe(expectedDuration);
+      expect(session._aecWarmupRemaining).toBe(expectedDuration ?? 0);
+    },
+  );
+
+  it.each([null, 0, 1500] as const)(
+    'preserves an explicit AEC warmup duration of %s for outbound SIP',
+    (duration) => {
+      const session = new AgentSession({ vad: null, aecWarmupDuration: duration });
+      const participant = {
+        info: { kind: ParticipantKind.SIP },
+        attributes: {},
+      } as RemoteParticipant;
+
+      session._onRoomIOParticipantLinked(participant);
+
+      expect(session.sessionOptions.aecWarmupDuration).toBe(duration);
+      expect(session._aecWarmupRemaining).toBe(duration ?? 0);
+    },
+  );
+
+  it('cancels AEC warmup that already started for outbound SIP', () => {
+    const session = new AgentSession({ vad: null });
+    const internals = session as AgentSessionInternals;
+    const timer = setTimeout(() => {}, 10_000);
+    internals._aecWarmupTimer = timer;
+    const clearTimeoutSpy = vi.spyOn(globalThis, 'clearTimeout');
+    const participant = {
+      info: { kind: ParticipantKind.SIP },
+      attributes: {},
+    } as RemoteParticipant;
+
+    session._onRoomIOParticipantLinked(participant);
+
+    expect(clearTimeoutSpy).toHaveBeenCalledWith(timer);
+    expect(internals._aecWarmupTimer).toBeNull();
+    clearTimeoutSpy.mockRestore();
+  });
+});
+
 describe('AgentSession.run', () => {
   it('forwards inputModality to generateReply', async () => {
     const session = new AgentSession();
diff --git a/agents/src/voice/agent_session.ts b/agents/src/voice/agent_session.ts
index bd0b5c47c..cb16f44c1 100644
--- a/agents/src/voice/agent_session.ts
+++ b/agents/src/voice/agent_session.ts
@@ -4,7 +4,12 @@
 import { type JsonObject, Struct } from '@bufbuild/protobuf';
 import { Mutex } from '@livekit/mutex';
 import { AgentSession as pb } from '@livekit/protocol';
-import type { AudioFrame, Room } from '@livekit/rtc-node';
+import {
+  type AudioFrame,
+  ParticipantKind,
+  type RemoteParticipant,
+  type Room,
+} from '@livekit/rtc-node';
 import { ThrowsPromise } from '@livekit/throws-transformer/throws';
 import type { TypedEventEmitter as TypedEmitter } from '@livekit/typed-emitter';
 import type { Context, Span } from '@opentelemetry/api';
@@ -120,6 +125,9 @@ import type {
 import { migrateLegacyOptions, stripUndefined } from './turn_config/utils.js';
 import { setParticipantSpanAttributes } from './utils.js';
 
+const SIP_RULE_ID_ATTR = 'sip.ruleID';
+const DEFAULT_AEC_WARMUP_DURATION = 3000;
+
 export interface AgentSessionUsage {
   /** List of usage summaries, one per model/provider combination. */
   modelUsage: Array<Partial<ModelUsage>>;
@@ -291,7 +299,7 @@ export type AgentSessionOptions<UserData = UnknownUserData> = {
   /**
    * Duration in milliseconds for AEC (Acoustic Echo Cancellation) warmup, during which
    * interruptions from audio activity are suppressed. Set to `null` to disable.
-   * @defaultValue 3000
+   * Defaults to 3000, or `null` for outbound SIP calls.
    */
   aecWarmupDuration?: number | null;
 
@@ -417,6 +425,7 @@ export class AgentSession<
   private idleReleased = new Event();
 
   private _aecWarmupTimer: NodeJS.Timeout | null = null;
+  private readonly _aecWarmupDurationExplicit: boolean;
 
   // Connection options for STT, LLM, and TTS
   private _connOptions: ResolvedSessionConnectOptions;
@@ -510,6 +519,7 @@ export class AgentSession<
   constructor(options: AgentSessionOptions<UserData> = {}) {
     super();
 
+    this._aecWarmupDurationExplicit = options.aecWarmupDuration !== undefined;
     const { agentSessionOptions: opts, legacyVoiceOptions } =
       migrateLegacyOptions<UserData>(options);
 
@@ -1665,6 +1675,23 @@ export class AgentSession<
     }
   }
 
+  /** @internal */
+  _onRoomIOParticipantLinked(participant: RemoteParticipant): void {
+    if (this._aecWarmupDurationExplicit) {
+      return;
+    }
+
+    const isOutboundSip =
+      participant.info.kind === ParticipantKind.SIP && !participant.attributes[SIP_RULE_ID_ATTR];
+    this.sessionOptions.aecWarmupDuration = isOutboundSip ? null : DEFAULT_AEC_WARMUP_DURATION;
+    this._aecWarmupRemaining = this.sessionOptions.aecWarmupDuration ?? 0;
+
+    if (isOutboundSip && this._aecWarmupTimer !== null) {
+      clearTimeout(this._aecWarmupTimer);
+      this._aecWarmupTimer = null;
+    }
+  }
+
   private _onUserInputTranscribed(ev: UserInputTranscribedEvent): void {
     if (ev.isFinal && this._userState !== 'speaking') {
       if (this._userState === 'away') {
diff --git a/agents/src/voice/room_io/room_io.ts b/agents/src/voice/room_io/room_io.ts
index b83471ade..4a955c5ce 100644
--- a/agents/src/voice/room_io/room_io.ts
+++ b/agents/src/voice/room_io/room_io.ts
@@ -259,6 +259,7 @@ export class RoomIO {
     }
 
     this.participantAvailableFuture.resolve(participant);
+    this.agentSession._onRoomIOParticipantLinked(participant);
   };
 
   private onParticipantDisconnected = (participant: RemoteParticipant) => {