select store_num, count(*) custcount from (
   select store_num, concat(area_code,phone_num), count(concat(area_code,phone_num))
   from orders_p
   where ord_date>='00000000' and ord_date<='20070128' and store_num=1
   group by concat(area_code,phone_num)
) ordpsub group by store_num

+--------+-------------+--------+--------+---------------+--------+---------+--------+--------+--------+
| id     | select_type | table  | type   | possible_keys | key    | key_len | ref    | rows   | Extra  |
+--------+-------------+--------+--------+---------------+--------+---------+--------+--------+--------+
| 2      | DERIVED     | orders_p| range  | PRIMARY,order_date,store_number,date_store| PRIMARY| 14      | [NULL] | 139250 | Using where; Using temporary; Using filesort|
| 1      | PRIMARY     | <derived2>| ALL    | [NULL]        | [NULL] | [NULL]  | [NULL] | 20720  | Using temporary; Using filesort|
+--------+-------------+--------+--------+---------------+--------+---------+--------+--------+--------+