I don't think any fax server I've ever seen can span Exchange sites... it installs as a gateway and they typically do lookups in Exchange via LDAP and are limited to finding users within the site. That's not to say there isn't any fax servers out there that work differently but all the one's I've seen have followed that basic pattern.
yeah, this is more of a question for the fax server folks - Unity just sees the faxes in the inbox and presents them over the phone and sends outbound fax by talking to the gateway - we don't care which server is installed or how they work so long as they have a known message class and conform to the standard fax gateway rules.
