A cell phone lookup is a service that allows users to find out who owns a particular cell phone based on the phone's number. There are several online directories that offer reverse lookup on cell numbers, as well as other methods of finding a particular cell's owner.
In general, cell phone number directories provide mixed results. Cell numbers are not maintained in a central network like landline numbers, but rather by individual cell phone service providers, so compiling them in a central database is challenging. Regardless, many services offer free reverse cell phone lookup, including the online database Spy Dialer.