docs(e911): add warning callout explaining fees and testing

Author: hey-august

fern/products/platform/pages/platform/phone-numbers/e911.mdx

@@ -16,6 +16,14 @@ E911 on SignalWire is self-service: create an emergency address, assign it to a
 and the number is provisioned for emergency calling. You can manage everything from the
 [Dashboard](#set-up-e911-in-the-dashboard) or the [REST API](#set-up-e911-with-the-rest-api).
 
+<Warning title="911 calls from unregistered numbers">
+Calling 911 from a number with no E911 address still connects, but the call routes to the
+national emergency call center, where operators must determine your location before help can be
+dispatched — delaying response. Each of these calls also incurs a $100 fee. Register an address
+before an emergency, and always dial 933, not 911, to
+[test your configuration](#test-your-e911-configuration).
+</Warning>
+
 ## How it works
 
 Emergency addresses live in your Space as a reusable collection, so one address can serve many phone numbers.