Update sip-trunk.mdx

Author: sahilsuman933

fern/advanced/sip/sip-trunk.mdx

@@ -100,7 +100,12 @@ 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).
+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}@<credential_id>.sip.vapi.ai` for Zadarma).
+
+<Warning>
+Note: Please ensure that you provide all the signaling IP addresses when creating the SIP trunk. Failure to do so will prevent proper whitelisting, which may result in encountering unauthorized 401 errors for inbound calls.
+</Warning>
+
 
 #### SIP REFER (Call Transfer)