The Limitations of FreeSwitch
FreeSWITCH is a B2BUA (back-to-back user agent). This means that it actually parses each of the SIP messages it receives. FreeSWITCH can not act as a proxy, for instance by forwarding SIP registrations to a registrar server. Of course, you could cluster multiple FreeSWITCH servers into sharing registrations using an ODBC data source :).
As far as what can be done with a B2BUA, the sky is the limit!