A secure and anonymous conference call set-up scheme is proposed for a portable cellular mobile system. The proposed scheme uses an identity-based concept and enables a mobile unit and a base station to directly authenticate each other by their public identity. It provides mobile subscribers with user identification privacy. Each mobile unit joining in this system can determine whether it is part of a conference call, but it cannot derive any further information about who else is also in the conference. That is to say, an anonymous property, accomplished through the aspect of the knapsack-like cipher mechanism, is achieved among the communicating mobiles. Furthermore, reauthentication in the course of hand-off is also discussed and the reauthentication procedure is performed through a privacy homomorphism mechanism. The time computation with an 8-bit microcontroller handset is acceptable for performing an anonymous conference call in such mobile systems. Therefore, our scheme is feasible and able to be implemented in the existing wireless exploration environment.