While you can't find someone's unlisted phone number, there are a number of white page and yellow page phone listings freely available online. One favorite is Langenberg.com, which combines several of them (InfoSpace, SwitchBoard, Verizon, Dex, etc) into one neat package.
With Langenberg, you can also do reverse lookups. In other words, you type in a phone number to find out who it belongs to. On that same page, you can do a lookup by street address.
To find company phone numbers, you can try Langenberg's yellow pages, or lookup the company in Hoover's, as they provide address and phone as well.