People Search Network
Sometimes, if you are lucky enough, you can also come across the cell phone number of people just by searching it through the search engines like google. Many people love to create their own websites for professional reasons. So, the search engines would automatically lead you to his page where you may find out his photo, contact address and cell phone number. The networking sites like facebook, orkut, mySpace etc. are some of the other sites from where you can stumble upon the cell phone number. People also become part of many groups and forums depending on their interests.
So, if you happen to know the hobbies and other interest of the person, then you can visit such groups and forums and may be able to contact him there and ask for his cell phone number after introducing yourself.