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Exchange 2007 to Exchange 2010 free/busy coexistence

There are a lot of articles out there on how to handle coexistence between Exchange 2003 and Exchange 2010. Yet there is no article that describes free busy coexistence between Exchange 2007 and Exchange 2010. What motivate me write this article is the lack of documentation from MS in this scenario. While there is good amount of articles on Exchange 2007 and Exchange 2010 CAS coexistence on EWS, Autodiscover, ActiveSync and others.

Our company is currently running Exchange 2007 and moving to Exchange 2010. We are also using F5s to loadbalance OWA, ActiveSync, Autodiscover and other services. I did all the prep work by building exchange 2010 CAS servers and creating the legacy.company.com DNS record and point it to Exchange 2010 CAS server(in our case, VIP on the F5s). Of course, first step is to flip the CAS server, then hub, the move mailboxes so on….Part of the first step in the migration plan to Exchange 2010, last week I flipped the switch on the DNS mail.company.com to point to Exchange 2010 CAS server. Then, went into every CAS server and set the autodiscover url(set-clientaccessserver) to point to the F5s. Also pointed EWS,AS and other virtual directories to point to the F5s(set-webservicesvirtualdirectory). Now, time to confirm if everything working as it should. A few coexistence tests I ran:

CAS Proxy test – login to Exchange 2007 mailbox from Exchange 2010 CAS (It works)
ActiveSync test -- login to Exchange 2007 mailbox from Exchange 2010 CAS (It works)
Autodiscover test – test Autodiscover config from Outlook client (It works)
Free/busy test – test free/busy from Outlook client (It doesn’t work)

I am confused as to why free/busy did not work since this is not documented anywhere in MS migration document. After troubleshooting for a few hours, here is what I found:
For free/busy to work Exchange 2007 to Exchange 2010 coexistence, you must set the webservices url on Exchange 2007 CAS servers to legacy.company.com. Below are the commands:

On Exchange 2007 CAS servers, run:
set-webservicesvirtualdirectory –internalurl legacy.company.com
set-webservicesvirtualdirectory –externalurl legacy.company.com