The optimization process and recommendations:

1. Avoid Calling Functions With Indexed Columns (query line: 32): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `call_datetime` is indexed, the index won’t be used as it’s wrapped with the function `DATE`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.
2. Mixed Order By Directions Prevents Index Use (query line: 36): The database will not use a sorting index (if exists) in cases where the query mixes ASC (the default if not specified) and DESC order. To avoid filesort, you may consider using the same order type for all columns. Another option that will allow you to switch one direction to another is to create a new reversed "sort" column (max_sort - sort) and index it instead.

The optimized query:

SELECT
        persontbl1._CREATION_DATE,
        persontbl2._TOP_LEVEL_AURI,
        persontbl2.RESP_CNIC,
        persontbl2.RESP_CNIC_NAME,
        persontbl1.MOB_NUMBER1,
        persontbl1.MOB_NUMBER2,
        schedule.id,
        schedule.call_datetime,
        schedule.enum_id,
        schedule.enum_change,
        schedule.status 
    FROM
        persontbl1 
    INNER JOIN
        persontbl2 
            ON (
                persontbl2._TOP_LEVEL_AURI = persontbl1._URI
            ) 
            AND (
                AGR_CONTACT = 1
            ) 
    LEFT JOIN
        SCHEDULE 
            ON (
                schedule.survey_id = persontbl1._URI
            ) 
            AND (
                SCHEDULE.status = 0
            ) 
            AND (
                DATE(SCHEDULE.call_datetime) <= CURDATE()
            ) 
    ORDER BY
        schedule.call_datetime IS NULL DESC,
        persontbl1._CREATION_DATE ASC