counts1.clj

(defn count-area-codes [inp-seq]
  (reduce (fn [m line]
            (let [phnum         (second (.split line "\t"))
                  [_ area-code] (first (re-seq #"\((\d+)\)" phnum))]
              (assoc m area-code (inc (get m area-code 0)))))
          {}
          inp-seq))

  (apply
   merge-with +
   (map (fn [inp-file]
          (count-area-codes (ds/read-lines inp-file)))
        (map str (filter #(.isFile %) (.listFiles (java.io.File. "working-dir/"))))))