If you forgot to do this during setup, or you just plain didn’t know what the external name was going to be and now addressbook lookups don’t work from the outside, there is hope!
LCSCMD to the rescue. You may find this under
\Program Files\Common Files\Microsoft Office Communications Server 2007\
The command line for changing or updating the reverse proxy FQDN is:
Lcscmd /web /action:updatepoolurls /externalwebfqdn:externalreverseproxyurl /poolname:yourpoolname
where externalreverseproxyurl should be in the form of external.contoso.com and yourpoolname is the