DB[:billing_data].insert([:column1, :column2],
 DB[:cdrs___c].
  select(:c__id, :c__variable_duration * :r__rate___cost).
  join(:rates___r){|j,lj,pjs| {:prefix.qualify(j)=>
    :substring.sql_function(:variable_destination_number.qualify(lj), 1, :length.sql_function(:prefix.qualify(j)))}}
)