Update call-forwarding.mdx

Author: sahilsuman933

fern/call-forwarding.mdx

@@ -186,6 +186,8 @@ Vapi supports two types of call transfers:
 
 To implement a warm transfer, add a `transferPlan` object to the `transferCall` tool syntax and specify the transfer mode.
 
+<Warning>Note: Warm transfer functionality is currently available only with Twilio-based telephony systems.</Warning>
+
 #### Modes of Warm Transfer
 
 #### 1. Warm Transfer with Summary