If you use a Microsoft Exchange Server 2007 Unified Messaging server to receive faxes, some fax communication sessions are dropped by the Unified Messaging server. Therefore, some faxes are missing.
The fax communication sessions may include empty data fields, which is ordinary. However, the Exchange Server 2007 Unified Messaging server does not recognize the empty data fields. Therefore, the Exchange Server 2007 Unified Messaging server ends the connection by sending a Session Initiation Protocol (SIP) "BYE" during the fax transmission.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.