I wrote an article here a while back on a product by XO called IP Flex. In it I listed an issue we were having with caller ID on tandem calls (call forward & EC500). I was notified by my Avaya Business Partner, Sunturn http://www.sunturn.com/, that Avaya released a fix for this issue that is included in Communication Manager 5.2.1. I have since upgraded to this release and, after enabling the recommended 'Special Applications', we are able to pass caller ID on tandem calls.
The special applications are as follows:
SA8870 (Page 6)
SA8931 (Page 7)