In many cases the easiest way to get a phone number is to look it up in your local telephone book or to call Information (411 or 555-1212). Also, you may have access to a CD-ROM with telephone numbers.
Alternatively, there are several free online telephone directories. I've gotten good results from Switchboard.com.
Other online telephone directories are available through infobel, Virtual Gumshoe, Superpages.com,
and/or The Ultimates. More free online directories are listed on Google.
If necessary, try fee-based services. Accurint, TLO, LocatePLUS, Lexis, Westlaw generally go beyond the free services.
Cell Phones: Cell phone numbers are generally private information, but some fee-based services sell them, including Intelius and Accurint. FreeCellPhoneTracer.com has a money back guarantee, which they will honor if you contact them. Alternatively, try the fee-based public records resources discussed above and hope a cell number pops up.
You can use the resources listed in the "Location Look-Up" section, below, to locate the city or region where the phone number was issued.
Foreign: There are many online directories for finding foreign telephone numbers -- you can find many of them using Infobel.com. For European businesses, also try EUROPAGES.com. If that doesn't work, I call the AT&T International Directory Assistance operator at 412-555-1515. There are Canadian telephone number directories on Accurint and Lexis (Public Records tab>Find a Phone Number>Canadian Phone Data).
For foreign country codes, direct diling prefixes and other information about dialing foreign numbers, visit DialCodesPlus.
Historical Information: The online sources seldom keep historical data. Your best is to find old telephone books, criss-cross directories or CD-ROM; these are often available in public libraries and genealogy libraries for the relevant town/city/county/region/etc. Haines Criss+Cross Directory subscribers can purchase historical information by calling the Haines Research Service (330-966-5550).
Location Look-Up: If you just need the country and city/town/etc. where a number rings, try the International Numbering Plans page by Spraakmaker Telecom FoneFinder, FoneFinder and/or FreeCellPhoneTracer.com. These can also tell you the name of the phone company and whether the number is active.
Reverse Directories: If you have a telephone number and want to find the related person or business you'll need a reverse directory, sometimes called a "criss-cross directory." Online, there are free options available through TheUltimates.com/white and Superpages.com. Intelius and Accurint have fee-based searches. In print and CD-ROM, the Haines Criss+Cross Directories cover business, government and organization phone numbers in larger metropolitan areas; subscribers can purchase historical information by calling the Haines Research Service (330-966-5550). Other criss-cross directories can be found in public and genealogy libraries for the relevant town/city/county/region/etc.
Search Detective has a reverse directory of cell-phone numbers.
TTY Directory: The deaf and hard-of-hearing have a special telephone directory for TTY users formally titled the TDI National Directory and Resource Guide, commonly known as "The Blue Book." The Blue Book is published by TDI (301-589-3786).
Unlisted Telephone Numbers: To get unlisted telephone numbers, try the fee-based databases discussed above. In addition to searching the phone directories consider the databases for property records, licensing and the databases discussed in the entry on "Finding People."