create table 'informix'.calls (
    call_id SERIAL not null,
    phone_num CHAR( 9 ) not null,
    zone_id CHAR( 10 ) not null,
    call_beg DATETIME YEAR TO SECOND not null,
    call_end DATETIME YEAR TO SECOND not null,
    kagent_phone_num NVARCHAR( 25 ) not null
)

SELECT EXTEND(call_end, MINUTE TO SECOND) - EXTEND(call_beg, MINUTE TO SECOND) FROM calls
WHERE (EXTEND(call_beg, HOUR TO SECOND) < EXTEND('07:59:59', HOUR TO SECOND) OR
      EXTEND(call_beg, HOUR TO SECOND) > EXTEND('23:00:00', HOUR TO SECOND))

  SUM(
    CASE
      WHEN
        EXTEND(call_beg, HOUR TO SECOND)
          BETWEEN DATETIME( 23 : 00 : 00 ) HOUR TO SECOND
                 AND DATETIME( 7 : 59 : 59 ) HOUR TO SECOND
        THEN call_end - call_beg

CREATE FUNCTION "informix".process_calls()
RETURNING CHAR( 9 ), CHAR( 10 ), INT, INT, INT

DEFINE _sqlcode, _ret INT;
DEFINE _phone_num CHAR( 9 );
DEFINE _zone_id CHAR( 10 );
DEFINE _b_time INT;
DEFINE _not_b_time INT;
DEFINE _beg_date, _end_date, _tmp_date DATETIME YEAR TO SECOND;
ON EXCEPTION SET _sqlcode
ROLLBACK WORK;
RETURN '', '',  0 ,  0 , _sqlcode;
END EXCEPTION;
SET LOCK MODE TO WAIT  5 ;

    LET _ret =  0 ;
    BEGIN WORK; 
    
          FOREACH
                 SELECT phone_num, zone_id, call_beg, call_end
                 INTO _phone_num, _zone_id, _beg_date, _end_date
                 FROM calls
                 
                 -- если разговор начался в ДНЕВНОЕ время, а закончился в НОЧНОЕ
                 IF ((EXTEND(_beg_date, HOUR TO SECOND)
                    BETWEEN DATETIME( 8 : 00 : 00 ) HOUR TO SECOND
                            AND
                            DATETIME( 22 : 59 : 59 ) HOUR TO SECOND))
                    AND
                    ((EXTEND(_end_date, HOUR TO SECOND)
                    BETWEEN DATETIME( 23 : 00 : 00 ) HOUR TO SECOND
                            AND
                            DATETIME( 7 : 59 : 59 ) HOUR TO SECOND)
                    ) THEN
                    LET _tmp_date = EXTEND(_beg_date, YEAR TO DAY) + EXTEND('22:59:59', HOUR TO SECOND);
                    CALL interval_in_sec(_beg_date, _tmp_date) RETURNING _b_time;
                    CALL interval_in_sec(_tmp_date, _end_date) RETURNING _not_b_time;
                    
                    RETURN _phone_num, _zone_id, _b_time, _not_b_time,  0  WITH RESUME;
                    
                 -- если разговор начался в ДНЕВНОЕ время и закончился также в ДНЕВНОЕ   
                 ELIF ((EXTEND(_beg_date, HOUR TO SECOND)
                      BETWEEN DATETIME( 8 : 00 : 00 ) HOUR TO SECOND
                              AND
                              DATETIME( 22 : 59 : 59 ) HOUR TO SECOND))
                      AND
                      ((EXTEND(_end_date, HOUR TO SECOND)
                      BETWEEN DATETIME( 7 : 59 : 59 ) HOUR TO SECOND
                              AND
                              DATETIME( 23 : 00 : 00 ) HOUR TO SECOND)
                    ) THEN
                    CALL interval_in_sec(_beg_date, _end_date) RETURNING _b_time;
                    LET _not_b_time =  0 ;
                    
                    RETURN _phone_num, _zone_id, _b_time, _not_b_time,  0  WITH RESUME;
                    
                 -- если разговор начался в НОЧНОЕ время, а закончился в ДНЕВНОЕ   
                 ELIF ((EXTEND(_beg_date, HOUR TO SECOND)
                      BETWEEN DATETIME( 23 : 00 : 00 ) HOUR TO SECOND
                              AND
                              DATETIME( 00 : 00 : 00 ) HOUR TO SECOND)
                      OR (EXTEND(_beg_date, HOUR TO SECOND)
                      BETWEEN DATETIME( 00 : 00 : 01 ) HOUR TO SECOND
                              AND
                              DATETIME( 7 : 59 : 59 ) HOUR TO SECOND))
                      AND
                      ((EXTEND(_end_date, HOUR TO SECOND)
                      BETWEEN DATETIME( 8 : 00 : 00 ) HOUR TO SECOND
                              AND
                              DATETIME( 22 : 59 : 59 ) HOUR TO SECOND)
                    ) THEN
                    LET _tmp_date = EXTEND(_beg_date, YEAR TO DAY) + EXTEND('7:59:59', HOUR TO SECOND);
                    CALL interval_in_sec(_beg_date, _tmp_date) RETURNING _not_b_time;
                    CALL interval_in_sec(_tmp_date, _end_date) RETURNING _b_time;
                    
                    RETURN _phone_num, _zone_id, _b_time, _not_b_time,  0  WITH RESUME;
                    
                 -- если разговор начался в НОЧНОЕ время и закончился также в НОЧНОЕ   
                 ELIF ((EXTEND(_beg_date, HOUR TO SECOND)
                      BETWEEN DATETIME( 23 : 00 : 00 ) HOUR TO SECOND
                              AND
                              DATETIME( 00 : 00 : 00 ) HOUR TO SECOND)
                      OR (EXTEND(_beg_date, HOUR TO SECOND)
                      BETWEEN DATETIME( 00 : 00 : 01 ) HOUR TO SECOND
                              AND
                              DATETIME( 7 : 59 : 59 ) HOUR TO SECOND))
                      AND
                      ((EXTEND(_end_date, HOUR TO SECOND)
                      BETWEEN DATETIME( 23 : 00 : 00 ) HOUR TO SECOND
                              AND
                              DATETIME( 00 : 00 : 00 ) HOUR TO SECOND)
                      OR (EXTEND(_beg_date, HOUR TO SECOND)
                      BETWEEN DATETIME( 00 : 00 : 01 ) HOUR TO SECOND
                              AND
                              DATETIME( 7 : 59 : 59 ) HOUR TO SECOND)
                    ) THEN
                    CALL interval_in_sec(_beg_date, _end_date) RETURNING _not_b_time;
                    LET _b_time =  0 ;
                    
                    RETURN _phone_num, _zone_id, _b_time, _not_b_time,  0  WITH RESUME;
                 END IF;
          END FOREACH;
          
    COMMIT WORK;
    RETURN '', '',  0 ,  0 , _ret;

END FUNCTION;

CALL interval_in_sec(_beg_date, _tmp_date) RETURNING _b_time;

LET _b_time = interval_in_sec(_beg_date, _tmp_date);

BEGIN WORK;
...
COMMIT WORK;