If all the information you have is a telephone number, you can find the phone company or carrier for the phone, as well as details about who owns the phone. This information is useful if you're being called by someone at a number you don't recognise. You might also be interested in finding out the carrier for a number of someone you know. Often, if they have the same carrier as you do, you can talk to that person for free.
Write down the number for which you want to find the carrier. If the number belongs to someone you know, you might already have it, but if you're trying to find out more information about a number you don't know, you'll need to write it down when your phone is called.
Choose a reverse phone lookup service to use. Most such services are free, but not all reverse phone lookups will tell you the carrier for the phone number. A few that will include PhoneBooks.com and Number-Finder.info (see Resources).
Visit the website for the reverse phone lookup service you chose. Type in the phone number you wrote down and click "Search."
Scroll through the available information to find the phone carrier. It will be listed next to "Phone Carrier" or "Company."