Do you have a phone number for a person that you believe is a public payphone number?  The Payphone Project has a public database of over 500,000 unlisted payphone number records that you can do a reverse payphone number search on – Reverse Payphone Number Lookup . 

If you don’t find the number on the payphone number search above, you can always try reversing the phone number on one of the major online search engines to see if the unlisted phone number can be found on any other indexed web sites.