Select where datetime is greater than other datetime which may not exist
Pull the COALESCE out of the subquery, to make it sargable:

        SELECT  min(eventtime) time_min
            FROM  `cel`
            WHERE  eventtime &gt; 
                COALESCE( ( SELECT  MAX(other_datetime)
                                   FROM  `call` ), 0)


And indexes:

call:  INDEX(other_datetime)
cel:   INDEX(eventtime)


That should be able to use both indexes to good advantage and take essentially zero time.

If there is an issue with 0 not being a date, try "0000-00-00"