D'oh. Wrong silence code.

ROM2/main_basic.c

@@ -236,7 +236,7 @@ void do_silence() {
 		send_string("511 Unknown silence duration." CRLF);
 		return;
 	}
-	send_string("500 Silence started." CRLF);
+	send_string("501 Silence started." CRLF);
 	return;
 }