The phone extension for Mailbox_999 is ext. 12345.
The user dials the Subscriber Access number from phone ext. 12345 and then enters his or her extension and PIN.
In this scenario, the Unified Messaging (UM) server responds with an "Invalid Mailbox Extension" error message.
Users can still use Outlook Voice Access (OVA) to access a shared mailbox. However, they must call from an extension of a non-shared mailbox. To work around this issue, the user must dial the Subscriber Access number from a different phone extension, such as ext. 78900. Then, enter ext. 12345 and the correct PIN. OVA can now successfully access Mailbox_999.