diff --git a/.changeset/sip-header-value-cap.md b/.changeset/sip-header-value-cap.md
new file mode 100644
index 000000000..4e8c35c2c
--- /dev/null
+++ b/.changeset/sip-header-value-cap.md
@@ -0,0 +1,6 @@
+---
+"github.com/livekit/protocol": patch
+"@livekit/protocol": patch
+---
+
+Raise the SIP header value cap from 1 KB to 4 KB so carrier headers like Twilio's `X-Twilio-CallToken` (Immutable Call Forwarding token with SHAKEN/STIR PASSporTs, >1 KB) pass `CreateSIPParticipant` validation
\ No newline at end of file
diff --git a/livekit/sip_validation.go b/livekit/sip_validation.go
index 814cd0c7c..da1f5a364 100644
--- a/livekit/sip_validation.go
+++ b/livekit/sip_validation.go
@@ -149,6 +149,12 @@ func init() {
 	headerValuesCharacters.AddUTF8()
 }
 
+// maxHeaderValueLength caps a single SIP header value. It is not an RFC limit
+// (RFC 3261 §7.1 bounds the message, not headers), but guards against abusive
+// or oversized values. Set above carriers' real-world maximums — Twilio's
+// X-Twilio-CallToken exceeds 1 KB — and well below the 8 KB UDP datagram bound.
+const maxHeaderValueLength = 4096
+
 // Required headers for SIP requests per RFC 3261 Section 8.1.1
 var RequiredRequestHeaders = map[string]bool{
 	"via":          true,
@@ -257,8 +263,13 @@ func ValidateHeaderValueResult(name, value string) ValidationResult {
 		return ValidationResult{}
 	}
 
-	if len(value) > 1024 {
-		return ValidationFailure(fmt.Errorf("header %s: value too long (max 1024 characters)", name))
+	// RFC 3261 caps the whole message (8 KB per §7.1 when UDP), not individual
+	// header values. Carriers do exceed 1 KB on a single header — e.g. Twilio's
+	// X-Twilio-CallToken carries an Immutable Call Forwarding token with
+	// SHAKEN/STIR + DIV PASSporTs that must be forwarded unchanged. 4096 keeps
+	// a single header from dominating a datagram while tolerating those.
+	if len(value) > maxHeaderValueLength {
+		return ValidationFailure(fmt.Errorf("header %s: value too long (max %d characters)", name, maxHeaderValueLength))
 	}
 
 	// Basic character validation - printable ASCII. We're stricter than the spec here - no UTF-8 for now
diff --git a/livekit/sip_validation_test.go b/livekit/sip_validation_test.go
index efccb6dbe..369182211 100644
--- a/livekit/sip_validation_test.go
+++ b/livekit/sip_validation_test.go
@@ -99,7 +99,8 @@ var ValidHeaderValues = []string{
 	"Header with unicode café",             // Unicode
 	"Header with unicode 世界",               // Unicode
 	"Header with unicode émojis 🎉",         // Unicode with emojis
-	strings.Repeat("a", 1024),              // max length
+	strings.Repeat("a", 1024),              // beyond carrier needs (e.g. X-Twilio-CallToken)
+	strings.Repeat("a", 4096),              // max length
 }
 
 // Note: These restrictions are NOT in RFC 3261 but are applied for security/performance
@@ -110,7 +111,7 @@ var InvalidHeaderValues = []string{
 	"Header with\x01control",        // control character
 	"Header with\x1Funit separator", // control character
 	"Header with\x7Fdelete",         // delete character
-	strings.Repeat("a", 1025),       // too long
+	strings.Repeat("a", 4097),       // too long
 }
 
 // testCaseName truncates a test case name to maxLen and adds dots with total size