Added voicemail audio messages docs (#634)

Author: ramsrib

fern/calls/voicemail-detection.mdx

@@ -453,6 +453,60 @@ Choose between two detection methods based on provider performance:
 
 ---
 
+## **Pre-recorded Audio Messages**
+
+Instead of text-to-speech, you can use pre-recorded audio files for your voicemail messages. Simply provide the URL to your audio file in the `voicemailMessage` property:
+
+<CodeBlocks>
+```json title="API Configuration"
+{
+  "name": "Sales Assistant with Audio Message",
+  "model": {
+    "provider": "openai",
+    "model": "gpt-4o"
+  },
+  "voicemailDetection": {
+    "provider": "vapi"
+  },
+  "voicemailMessage": "https://example.com/sales-voicemail.mp3"
+}
+```
+```typescript title="TypeScript SDK"
+const assistant = await vapi.assistants.create({
+  name: "Sales Assistant with Audio Message",
+  model: {
+    provider: "openai",
+    model: "gpt-4o"
+  },
+  voicemailDetection: {
+    provider: "vapi"
+  },
+  voicemailMessage: "https://example.com/sales-voicemail.wav"
+});
+```
+```python title="Python SDK"
+assistant = client.assistants.create(
+    name="Sales Assistant with Audio Message",
+    model={
+        "provider": "openai",
+        "model": "gpt-4o"
+    },
+    voicemail_detection={
+        "provider": "vapi"
+    },
+    voicemail_message="https://example.com/sales-voicemail.mp3"
+)
+```
+</CodeBlocks>
+
+**Supported formats**: `.wav` and `.mp3` files
+
+<Tip>
+Pre-recorded audio messages provide consistent quality and pronunciation, especially useful for brand-specific messaging or complex information like phone numbers and website URLs.
+</Tip>
+
+---
+
 ## **Disabling Voicemail Detection**
 
 To completely disable voicemail detection for your assistant, set the `voicemailDetection` property to `"off"`:
@@ -596,6 +650,8 @@ When voicemail detection is disabled, your assistant will continue the conversat
 
 By using Vapi's detection system, you'll avoid the common pitfalls of voicemail detection, while creating a **faster, smarter, and more professional experience** for your users.
 
+
+
 ## **Related Documentation**
 
 - **[Voicemail Tool](/tools/voicemail-tool)** - Alternative assistant-controlled voicemail approach for maximum flexibility

fern/tools/voicemail-tool.mdx

@@ -131,6 +131,8 @@ assistant = client.assistants.create(
 
 Define the voicemail message in the tool configuration:
 
+### **Text-to-Speech Messages**
+
 ```json
 {
   "messages": [
@@ -146,9 +148,64 @@ Define the voicemail message in the tool configuration:
 Use template variables like `{{company}}`, `{{message}}`, and `{{phone}}` to make your voicemail messages dynamic while keeping them consistent.
 </Tip>
 
+### **Pre-recorded Audio Messages**
+
+For consistent quality and pronunciation, use pre-recorded audio files by providing the URL in the `content` field:
+
+```json
+{
+  "messages": [
+    {
+      "type": "request-start",
+      "content": "https://example.com/voicemail.mp3"
+    }
+  ]
+}
+```
+
+**Supported formats**: `.wav` and `.mp3` files
+
+<Note>
+Pre-recorded audio messages are ideal for brand-specific messaging or when you need precise pronunciation of phone numbers, website URLs, or company names.
+</Note>
+
 ## Advanced Examples
 
-### Dynamic voicemail with context
+### **Pre-recorded Audio Example**
+
+Using pre-recorded audio for professional voicemail messages:
+
+```json
+{
+  "model": {
+    "provider": "openai",
+    "model": "gpt-4o",
+    "messages": [
+      {
+        "type": "system",
+        "content": "You are a sales representative calling prospects. If you reach voicemail, use the leave_voicemail tool to play our professional pre-recorded message."
+      }
+    ],
+    "tools": [
+      {
+        "type": "voicemail",
+        "function": {
+          "name": "leave_voicemail",
+          "description": "Leave a professional pre-recorded voicemail message"
+        },
+        "messages": [
+          {
+            "type": "request-start",
+            "content": "https://example.com/professional-sales-voicemail.mp3"
+          }
+        ]
+      }
+    ]
+  }
+}
+```
+
+### **Dynamic voicemail with context**
 
 ```json
 {