diff --git a/API-Protocol.md b/API-Protocol.md
index 41384dd..baf5d27 100644
--- a/API-Protocol.md
+++ b/API-Protocol.md
@@ -45,6 +45,7 @@ The client submits the verification code it received via voice or SMS to the ser
 1. `401` badly formatted `basic_auth`.
 1. `403` incorrect `verification_code`.
 1. `413` rate limit exceeded.
+1. `417` number already registered
 
 ## Registering an APN or GCM id