updated general sip instruction

Signed-off-by: sahil suman <[email protected]>

Author: sahilsuman933

fern/advanced/sip/sip-trunk.mdx

@@ -10,10 +10,19 @@ SIP trunking replaces traditional phone lines with a virtual connection over the
 
 Vapi supports multiple SIP trunk configurations, including:
 
-- **Telnyx**: Uses SIP gateway domain (e.g., sip.telnyx.com) with IP-based authentication. May require a tech prefix for outbound calls.
+- **Telnyx**: Uses SIP gateway domain (e.g., sip.telnyx.com) with IP-based authentication. 
 - **Zadarma**: Uses SIP credentials (username/password) with its SIP server (e.g., sip.zadarma.com).
 - **Custom "BYO" SIP Trunk**: Allows integration with any SIP provider. You simply provide the SIP gateway address and the necessary authentication details.
 
+<Warning>
+Our IP Addresses are: 
+
+- 44.229.228.186/32
+- 44.238.177.138/32
+
+However, we generally don't recommend IP-based authentication for SIP trunks as it can lead to routing issues. Since our servers are shared by many customers, if your telephony provider has multiple customers using IP-based authentication, calls may be routed incorrectly. IP-based authentication works reliably only when your SIP provider offers a unique termination URI or a dedicated SIP server for each customer, as is the case with Plivo and Twilio integrations.
+</Warning>
+
 ## 2. Setup Process Overview
 
 To set up a SIP trunk in Vapi, follow these steps:
@@ -92,3 +101,27 @@ POST https://api.vapi.ai/call/phone
 #### Inbound Call Test
 
 If inbound routing is configured, call your phone number from an external line. Ensure your provider forwards calls to the correct SIP URI (e.g., `{phoneNumber}@sip.vapi.ai` for Zadarma).
+
+#### SIP REFER (Call Transfer)
+
+If you need to transfer a call to another number, you will need to add a SIP Transfer based call forwarding where transfer number will look like this: sip:[email protected]
+
+Example: sip:[email protected]
+
+Example tool configuration required for SIP REFER:
+
+```json
+{
+      "type": "transferCall",
+      "destinations": [
+        {
+          "type": "sip",
+          "sipUri": "sip:[email protected]"
+        }
+      ]
+    }
+```
+
+<Info>You might need to enable SIP REFER in your SIP provider to allow this.</Info>
+
+