XML phonebook for iOS ?
Out of the box, iOS supports CardDAV and LDAP.  There's no native way to import an XML file for contacts.

I have (in the past) set up an LDAP server (Solaris based) specifically for this scenario - I was connecting Nortel VoIP phones that supported a contact list with folks who needed access to this list but weren't on our Exchange Server (which had this list by default).

You may be able to set up some sort of synchronization between your phone system and the CardDAV or the LDAP server which will keep the contacts updated.  It may have this capability built in, but given that you can get an XML document of the list as output, creating a script to update a CardDAV/LDAP server would be pretty straight forward.