Is this a bug, or am I loosing it

I am getting weird results from a NOT IN and NOT EXISTS query. Could this be because of NULL's or is it a bug?

XX is the child table. YY is the parent table. There are some NULL's in XX.PH column whereas there are no NULL's in YY.PHONE_NUMBER.

DEVDB 1{MYACCT}> desc xx

 Name                         Null?    Type
 ---------------------------- -------- ------------------
 PH                                    NUMBER(10)

DEVDB 1{MYACCT}> desc yy
 Name                         Null?    Type
 ---------------------------- -------- ------------------
 PHONE_NUMBER                 NOT NULL NUMBER(10)

DEVDB 1{MYACCT}> select count(*) from yy; JAY SAYS {ENTER} ...   COUNT(*)

    688431

1 row selected.

Elapsed: 00:00:00.54
DEVDB 1{MYACCT}> select count(*) from xx; JAY SAYS {ENTER} ...   COUNT(*)

    152892

1 row selected.

Elapsed: 00:00:00.36
DEVDB 1{MYACCT}> SELECT phone_number FROM yy WHERE phone_number NOT IN ( SELECT ph FROM xx);

no rows selected

Elapsed: 00:00:28.93
DEVDB 1{MYACCT}>
DEVDB 1{MYACCT}> SELECT COUNT(phone_number) FROM yy WHERE NOT EXISTS (SELECT 1 FROM xx WHERE ph = phone_number); JAY SAYS {ENTER} ... COUNT(PHONE_NUMBER)

             666936

1 row selected.

Elapsed: 00:00:01.46
DEVDB 1{MYACCT}> SELECT count(phone_number) FROM yy WHERE phone_number NOT IN ( SELECT ph FROM xx); JAY SAYS {ENTER} ... COUNT(PHONE_NUMBER)

                  0

1 row selected.

Elapsed: 00:00:31.83

DEVDB 1{MYACCT}> SELECT count(phone_number) FROM yy WHERE phone_number NOT IN ( SELECT ph FROM xx WHERE ph IS NOT NULL); JAY SAYS {ENTER} ... COUNT(PHONE_NUMBER)

             666936

Regards,
J

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