Our organisation has Unified Communications implemented using Exchange 2007 SP1 and OCS 2007 R2.    We were using the Exchange AutoAttendants to route calls to OCS endpoints. 

Issue

When an external PSTN user, calling from an anonymous phone line (Caller ID disabled), would select an AutoAttendant menu option that forwarded him to an OCS user, the Exchange AA would not make the transfer and announce this to the external PSTN caller:  "transfer failed".

Workaround

As a workaround we programmed our gateway to replace all empty caller ID strings by a fake number, e.g. "+32999999".  Using the fake number, transfers did succeed.

Solution

We worked with Microsoft support on this, and apparantly it is a bug in Exchange 2007.  We just received a hotfix that resolved the issue.  The hotfix is identified by KB975050. 

Versions

Although our hotfix has been designed for 2007 SP1, the issue probably also occurs in 2007 SP2.  Originally we decided with the support engineer to fix it in SP2, but in the end we opted for a hotfix on our currently installed environment.

Should you experience the same issue,  you can ask Microsoft support for a hotffix using the KB number.  At the moment of writing this post, the fix is not yet announced on the web, but I will post a link when it becomes available.