diff --git a/src/ValueObjects/Voice/Dial.php b/src/ValueObjects/Voice/Dial.php
index 12e2c23..013cb3b 100644
--- a/src/ValueObjects/Voice/Dial.php
+++ b/src/ValueObjects/Voice/Dial.php
@@ -46,7 +46,7 @@ public function build(): string
         }
 
         if ($this->ringBackTone) {
-            $options .= " ringBackTone=\"{$this->ringBackTone}\"";
+            $options .= " ringbackTone=\"{$this->ringBackTone}\"";
         }
 
         if ($this->callerId) {