diff --git a/extensions/voice-call/src/config.ts b/extensions/voice-call/src/config.ts
index 9721cf380..2d1494c78 100644
--- a/extensions/voice-call/src/config.ts
+++ b/extensions/voice-call/src/config.ts
@@ -5,6 +5,7 @@ import {
   TtsProviderSchema,
 } from "openclaw/plugin-sdk/voice-call";
 import { z } from "zod";
+import { deepMergeDefined } from "./deep-merge.js";
 
 // -----------------------------------------------------------------------------
 // Phone Number Validation
@@ -376,45 +377,7 @@ function normalizeVoiceCallTtsConfig(
     return undefined;
   }
 
-  return TtsConfigSchema.parse({
-    ...(defaults ?? {}),
-    ...(overrides ?? {}),
-    modelOverrides:
-      defaults?.modelOverrides || overrides?.modelOverrides
-        ? {
-            ...(defaults?.modelOverrides ?? {}),
-            ...(overrides?.modelOverrides ?? {}),
-          }
-        : undefined,
-    elevenlabs:
-      defaults?.elevenlabs || overrides?.elevenlabs
-        ? {
-            ...(defaults?.elevenlabs ?? {}),
-            ...(overrides?.elevenlabs ?? {}),
-            voiceSettings:
-              defaults?.elevenlabs?.voiceSettings || overrides?.elevenlabs?.voiceSettings
-                ? {
-                    ...(defaults?.elevenlabs?.voiceSettings ?? {}),
-                    ...(overrides?.elevenlabs?.voiceSettings ?? {}),
-                  }
-                : undefined,
-          }
-        : undefined,
-    openai:
-      defaults?.openai || overrides?.openai
-        ? {
-            ...(defaults?.openai ?? {}),
-            ...(overrides?.openai ?? {}),
-          }
-        : undefined,
-    edge:
-      defaults?.edge || overrides?.edge
-        ? {
-            ...(defaults?.edge ?? {}),
-            ...(overrides?.edge ?? {}),
-          }
-        : undefined,
-  });
+  return TtsConfigSchema.parse(deepMergeDefined(defaults ?? {}, overrides ?? {}));
 }
 
 export function normalizeVoiceCallConfig(config: VoiceCallConfigInput): VoiceCallConfig {
diff --git a/extensions/voice-call/src/deep-merge.ts b/extensions/voice-call/src/deep-merge.ts
new file mode 100644
index 000000000..b889ec14e
--- /dev/null
+++ b/extensions/voice-call/src/deep-merge.ts
@@ -0,0 +1,23 @@
+const BLOCKED_MERGE_KEYS = new Set(["__proto__", "prototype", "constructor"]);
+
+export function deepMergeDefined(base: unknown, override: unknown): unknown {
+  if (!isPlainObject(base) || !isPlainObject(override)) {
+    return override === undefined ? base : override;
+  }
+
+  const result: Record<string, unknown> = { ...base };
+  for (const [key, value] of Object.entries(override)) {
+    if (BLOCKED_MERGE_KEYS.has(key) || value === undefined) {
+      continue;
+    }
+
+    const existing = result[key];
+    result[key] = key in result ? deepMergeDefined(existing, value) : value;
+  }
+
+  return result;
+}
+
+function isPlainObject(value: unknown): value is Record<string, unknown> {
+  return Boolean(value) && typeof value === "object" && !Array.isArray(value);
+}
diff --git a/extensions/voice-call/src/telephony-tts.ts b/extensions/voice-call/src/telephony-tts.ts
index da8e5f71a..f753a69f1 100644
--- a/extensions/voice-call/src/telephony-tts.ts
+++ b/extensions/voice-call/src/telephony-tts.ts
@@ -1,5 +1,6 @@
 import type { VoiceCallTtsConfig } from "./config.js";
 import type { CoreConfig } from "./core-bridge.js";
+import { deepMergeDefined } from "./deep-merge.js";
 import { convertPcmToMulaw8k } from "./telephony-audio.js";
 
 export type TelephonyTtsRuntime = {
@@ -20,8 +21,6 @@ export type TelephonyTtsProvider = {
   synthesizeForTelephony: (text: string) => Promise<Buffer>;
 };
 
-const BLOCKED_MERGE_KEYS = new Set(["__proto__", "prototype", "constructor"]);
-
 export function createTelephonyTtsProvider(params: {
   coreConfig: CoreConfig;
   ttsOverride?: VoiceCallTtsConfig;
@@ -79,28 +78,5 @@ function mergeTtsConfig(
   if (!base) {
     return override;
   }
-  return deepMerge(base, override);
-}
-
-function deepMerge<T>(base: T, override: T): T {
-  if (!isPlainObject(base) || !isPlainObject(override)) {
-    return override;
-  }
-  const result: Record<string, unknown> = { ...base };
-  for (const [key, value] of Object.entries(override)) {
-    if (BLOCKED_MERGE_KEYS.has(key) || value === undefined) {
-      continue;
-    }
-    const existing = (base as Record<string, unknown>)[key];
-    if (isPlainObject(existing) && isPlainObject(value)) {
-      result[key] = deepMerge(existing, value);
-    } else {
-      result[key] = value;
-    }
-  }
-  return result as T;
-}
-
-function isPlainObject(value: unknown): value is Record<string, unknown> {
-  return Boolean(value) && typeof value === "object" && !Array.isArray(value);
+  return deepMergeDefined(base, override) as VoiceCallTtsConfig;
 }