Add SIP URI transfer to call features (#767)

Author: sahilsuman933

fern/calls/call-features.mdx

@@ -129,6 +129,21 @@ curl -X POST 'https://aws-us-west-2-production1-phone-call-websocket.vapi.ai/742
 }'
 ```
 
+You can also transfer to a SIP URI:
+
+```bash
+curl -X POST 'https://aws-us-west-2-production1-phone-call-websocket.vapi.ai/7420f27a-30fd-4f49-a995-5549ae7cc00d/control' 
+-H 'content-type: application/json' 
+--data-raw '{
+  "type": "transfer",
+  "destination": {
+    "type": "sip",
+    "sipUri": "sip:[email protected]"
+  },
+  "content": "Testing transfer call."
+}'
+```
+
 ### 6. Handoff Call
 Handoff the call to a different assistant.