Telephone numbers are listed in many different places. Phone books, business cards, newspaper advertisements and flyers are among them. Locating numbers will require planning and research to find the best source for the ones you need.
Identify what kind of phone number you are looking for. Think about whether it belongs to a business, an individual, an association or, perhaps, an organisation. This will help you figure out where to find it. For example, if you are looking for the phone number of a residence in Slovakia, you would look in Slovak Telecom's online phone directory. (See Reference 1.)
Narrow the list of possible sources that would contain the phone number. For example, if you are looking for the number of a company in Hong Kong, you aren't going to find it in your local Yellow Pages or in your church's phone directory. You will also be more likely to find the number of your state's parks and wildlife department on the department's website than you would in the blue government/social-resources pages of your phone book.
Look in publications by, or about, the entity whose phone number you are looking for. Phone books in print and online, business cards, flyers on the notice boards of local coffee shops, news releases, volunteer requests sent out in e-mails, and many other sources will likely have the number.
Ask your local library's research person for ideas if you cannot locate a source for the phone number you need. They may be able to help you find another way that you haven't been able to identify.