I'm having a problem with a computer class Microsoft Access related assignment, can you help me?

I had this query assignment where I had to show all the people who didn't live in a particular town. I looked through the book and couldn't seem to find the solution, so I simply inputed into the criteria section NOT "town name" and it worked as desired. Sadly I'm in a similar boat now and can't find the answer in the book either. The assignment is to show only those who do not have a telephone number. There are two listed without telephone numbers, but the same critera NOT "telephone number" shows only those with a telephone number. I tried enter Telephone Number in the critera, but it showed me a blank sheet with no info save the headers. Any info would be appreciated. I'm sure it's in the text and I just can't find it.