timestamp with timezone issue with TZR vs TZH:TZM
You could define your table like this.

CREATE TABLE TEST_A (
    COLUMN_A  
        TIMESTAMP(6) WITH TIME ZONE
        DEFAULT SYSTIMESTAMP AT TIME ZONE '+00:00' 
        NOT NULL,
        CHECK (EXTRACT(TIMEZONE_REGION FROM COLUMN_A) = 'UNKNOWN')
    );

Table created.

INSERT INTO TEST_A VALUES (TO_TIMESTAMP_TZ('19-JUL-18 01.53.16.381566000 PM +00:00', 'DD-MON-RR HH.MI.SSXFF AM TZH:TZM'));

1 row created.

INSERT INTO TEST_A VALUES (TO_TIMESTAMP_TZ('19-JUL-18 01.53.16.381566000 PM UTC', 'DD-MON-RR HH.MI.SSXFF AM TZR'));

&gt;&gt; INSERT INTO TEST_A VALUES (TO_TIMESTAMP_TZ('19-JUL-18 01.53.16.381566000 PM UTC', 'DD-MON-RR HH.MI.SSXFF AM TZR'))
Error at line 11
ORA-02290: check constraint (SYS_C00241787) violated


Or you create a table with trigger:

CREATE TABLE TEST_B (
    COLUMN_A  
        TIMESTAMP(6) WITH TIME ZONE
        DEFAULT SYSTIMESTAMP AT TIME ZONE '+00:00' 
        NOT NULL
    );

CREATE OR REPLACE TRIGGER BUIR_TEST_B
    BEFORE INSERT OR UPDATE ON TEST_B
    FOR EACH ROW
BEGIN
    IF EXTRACT(TIMEZONE_REGION FROM :NEW.COLUMN_A) &lt;&gt; 'UNKNOWN' THEN
        :NEW.COLUMN_A := :NEW.COLUMN_A AT TIME ZONE TO_CHAR(:NEW.COLUMN_A, 'tzh:tzm');
    END IF;
END;
/


However, I don't believe this would be a proper solution. You may face issues when you work with time zones having daylight saving times.