Calendar integration is probably the most mysterious part of CUPS.
There are many catchas in Calendar integration due to the following:
1) CUPS uses WebDAV protocol to query calendar, which is a pretty old-school protocol and has many limitations. Microsoft recommend developers use EWS (Exchange Web Service) for better compatibility and features. Cisco will eventually change to EWS. But no ETA yet.
2) CUPS requires HTTPS (TLS/SSL) connection between CUPS and Exchange. This adds more complexity to the picture because you have to deal with CA, certificates, etc.
3) Exchange authentication and permissions
Exchange (OWA/IIS) has many authentication methods (FBA, classic, NTLM, etc.). Exchange also has two different sets of permissions: AD permission and Mailbox permission.
It's impossible to elaborate all scenarios in this blog. But here are some recommendations:
1) Avoid "2003/2007 mixed mode"
More likely than not, it's not going to work with CUPS due to limitation of WebDAV.
2) Avoid any firewall (especially MSFT ISA server) between CUPS and Exchange.
3) Avoid any load-balancer or traffice directory between CUPS and Exchange.
4) Try to disable FBA (Form Based Authentication) for troubleshooting purpose.
5) If you don't care about certificates, use "makecert.exe" utility to create self-signed certificate for Exchange. See http://www.lulu.com/content/5552336 for details.
6) Make sure you set your meeting status to "Busy" for testing. Make sure you set it to "whole day event" to avoid time zone glitches. (be aware, when you set it to "whole day event", the status in outlook will be "FREE" by default)