You will need to do something via <executeSQLQuery> for this.

Check the Data Dictionary for the tables and fields, but if 'Unassigned DNs' are DNs which do not appear on any device, something like this might work:

select dnorpattern from numplan where numplan.pkid not in (select fknumplan from devicenumplanmap)

Just so you know, it was my char array buffer size.

I also modified the sql query to only return the extensions starting with 5 or 3.

Might be useful to someone else, so here it is.
select dnorpattern from numplan where numplan.pkid not in (select fknumplan from devicenumplanmap) and (substring(dnorpattern FROM 1 FOR 1) = '5' or substring(dnorpattern FROM 1 FOR 1) = '3')

Thanks again!
-Alex