A method and apparatus are disclosed for preventing unauthorized access to a restricted item using a cellular telephone that has been previously associated with a user, for example, during a registration process. The user is initially identified, for example, by entering a password, and a one-time pseudo-random token is provided to the user using a first communication channel. The user is requested to dial a telephone number associated with an access control service using a cellular telephone that has been previously associated with the user and enter the assigned token. The user obtains access to the restricted item if the assigned token is entered from a cellular telephone having a serial number that has been previously associated with the particular user.