add more docs

Author: ramsrib

fern/call-forwarding.mdx

@@ -99,6 +99,19 @@ The `transferCall` tool includes a list of destinations and corresponding messag
 }
 ```
 
+You can also specify the `extension` parameter to forward the call to an extension.
+
+```json
+    "destinations": [
+        {
+            "type": "number",
+            "number": "+1234567890",
+            "extension": "4603",
+            "message": "I am forwarding your call to Department A. Please stay on the line."
+        }
+    ]
+```
+
 ### 2. Using the `transferCall` Function
 
 When the assistant needs to forward a call, it uses the `transferCall` function with the appropriate destination:
@@ -312,7 +325,7 @@ In this mode, Vapi executes TwiML instructions on the destination call leg befor
   - Provide the TwiML instructions in the `twiml` property.
   - Supports only `Play`, `Say`, `Gather`, and `Pause` verbs.
   - Maximum TwiML length is 4096 characters.
-  - TwiML must be provided as a single-line string without line breaks or tabs.
+  - TwiML must be provided as a single-line string without line breaks or tabs, and must be a valid XML string. For example: `<Say>Hello</Say>` is valid, but `<Say>Hello\n</Say>` is not.
 
 - **Example:**
 
@@ -349,4 +362,7 @@ Here is a full example of a `transferCall` payload using the warm transfer with
 }
 ```
 
-**Note:** In all warm transfer modes, the `{{transcript}}` variable contains the full transcript of the call and can be used within the `summaryPlan`.
+**Notes:**
+
+- In all warm transfer modes, the `{{transcript}}` variable contains the full transcript of the call and can be used within the `summaryPlan`.
+- For more details about transfer plans and configuration options, please refer to the [transferCall API documentation](https://docs.vapi.ai/api-reference/tools/create#request.body.transferCall.destinations.number.transferPlan)