A system and method for automatically verifying a telephone number proffered by a user of an application. During registration or installation of the application, client software is installed on the user's mobile device (e.g., a smart phone, a tablet computer). One of a pool of telephone numbers for outgoing calls is selected, and a call is initiated to the proffered number by the system. The software executing on the user's device detects the incoming call, extracts caller identification data (i.e., the originating telephone number), formats a data communication (e.g., an http request) to include the data, and sends it to the system. The system receives the data communication and compares the data with the originating number. If they match, the user's telephone number is verified. The outgoing call from the system may be canceled before it rings at the device if the data communication is received in time.