Use a caller identification application such as TrueCaller to determine who is calling from a cellphone number. Caller identification applications require mobile Internet connectivity to display caller information.
Truecaller works by aggregating data from directory services, and further complements this with phone book crowdsourced information from individual users of the application. Download and install the TrueCaller application on a compatible Internet-enabled cellphone to find out who is calling from an unknown number.
Most caller identification applications provide live caller identification as well as caller search services. To find out details of an earlier unknown caller, enter the number into the application's search bar to get the name and location of the caller. While user phone book data is not uploaded or made public, caller identification applications search through all devices on which the application is installed to identify the caller.
If your phone does not support a caller identification application, perform a manual trace using Google and other search engines. Cellphone numbers are not included in reverse phone number look-up services. Therefore, use people search services such as Spokeo or Intellius to trace out the number. If all options fail, simply call the number or message the caller to find out who responds.