You upgrade to Microsoft Exchange Server 2013 Cumulative Update 1 (CU1).
Exchange Server 2013 has the Unified Messaging (UM) language pack installed in addition to English (US).
The UM language pack is the RTM version.
UM Auto Attendant is speech enabled and is configured to use a language pack that is not English (US).
In this scenario, when a caller calls UM Auto Attendant, UM Auto Attendant hangs up after it plays an error message in the configured language. The error message that it plays translates to "Please call back later. Good-bye."
The issue occurs because the CU1 version of Exchange 2013 Unified Messaging cannot generate a grammar file by using the RTM version of the language pack. Therefore, all Unified Messaging operations that rely on a grammar file will fail.
To resolve this issue, install the Exchange Server 2013 CU1 UM Language Pack add-in.