Exchange 2007 EWS Availability Service - Free/Busy lookup not working

Free/Busy lookups are not working in my enviroment if i try to look up a mailbox in a different AD site.

I can lookup the free/busy information using Office 2007 or 2010 using EWS for any mailbox in the site my mailbox is in.

If i try to lookup theFree/Busy info for a mailbox who is in a different AD site it comes up unavailble. I must be missing some small setting to make this work but i have had no luck looking it up on the internet so far.

Any help is appreciated. Let me know if you need more details on anything.
Who is Participating?
Thanks for the update

It would appear that there is an issue with web services on Site B
Verify that the EWS virtual directory is configured with windows authentication
Also verify that the external URL for EWS is empty
Set-WebServicesVirtualDirectory SiteBCas\ews* -ExternalUrl $null -WindowsAuthentication:$True

Try browsing to the internal URL for Site B Cas on client
PoloNWSecurityAuthor Commented:
Update to issue: If i use a mailbox in the site that currently is not working i cannot view free/busy in that site but i can successfully see free/busy info in the first site which is working.

Site A - successful lookups in own site but cannot view site B

Site B -  successful lookups in site A but cannot view info in own site
is the entire environment exchange 2007 or is there a mix with 2003
can we get more detail for the exchange design

ad site of the user should have no impact when using ews

PoloNWSecurityAuthor Commented:
It is mixed but there is only a handful of users on 03 at this moment and none of them are an issue.

There is 2 AD sites each with Exchange 2007 servers.

Site A - 2 x HUB/CAS servers 4 x Mailbox Servers

Site B - 2 x HUB/CAS 1 x Mailbox Servers

Site A has the internet facing CAS and proxies to Site B for OWA and ActiveSync.
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