NULL is driving me nuts!

In my database, these three fields are defined as SMALLINT:
phone2_ac, phone2_ex, phone2_num

When inserting a recored into the database, I found that if my form fields for these were blank, this was what I had to do:
$phone2ac = 'NULL';
$phone2ex = 'NULL';  //if field is text, then you don't use the quotes
$phone2num = 'NULL';
I then used $phone2ac, $phone2ex, and $phone2num in my INSERT statement. If I omitted the quotes from 'NULL', I got an error on the insert.
My insert was like this:
INSERT INTO tblmine phone2_ac, phone2_ex, phone2_num VALUES ($phone2ac, $phone2ex, $phone2num);

All fine so far. In my database, the values are NULL.

Now, I do this:

if (!is_null($phone2ac) || !is_null($phone2ex) || !is_null($phone2num))  {
      $body .= "Other Phone: " . $phone2ac . "-" . $phone2ex . "-" . $phone2num . "\n"; }

When all three fields are null, $body is getting set. I do not understand why?

It's been a long day ...