diff --git a/fern/advanced/sip/sip.mdx b/fern/advanced/sip/sip.mdx
index 9ddd8419a..2b62c1813 100644
--- a/fern/advanced/sip/sip.mdx
+++ b/fern/advanced/sip/sip.mdx
@@ -8,6 +8,12 @@ slug: advanced/sip
 
 This guide shows you how to set up and test SIP calls to your Vapi assistant using any SIP client or softphone. You'll create an assistant, assign it a SIP phone number, and make a call using a SIP URI. You can also pass template variables via SIP headers.
 
+<Warning>
+  A Vapi SIP phone number is for **inbound calls only**. Because it is created with a `sipUri` and no area code, it has no PSTN phone number of its own, so there is no number for it to place an outbound call from. Outbound calls fail immediately with `call.start.error-get-transport` and the message `Vapi Numbers Can't Dial Outbound Yet.`
+
+  To place outbound calls, [import a number from a supported provider](/phone-numbers/import-twilio) or [connect your own SIP trunk](/advanced/sip/sip-trunk). A SIP trunk is also what you need to call international destinations.
+</Warning>
+
 ## Choose your Vapi region
 
 Use the API base URL, private key, and SIP host from the same Vapi region. Do not create a SIP phone number in one region with a SIP URI from another region.