Abstract: A mobile telephone incorporating the invention includes in its transmitter circuitry a system for preventing unauthorized calls from being transmitted by the telephone. The system ensures that the telephone transmit only service and call requests that include the electronic serial number ("ESN") which is permanently stored in the telephone by the manufacturer, in an unalterable ESN memory. The system in one embodiment includes ESN detection circuitry which determines the bit position of an ESN in a service or call request and either compares the ESN in the request with the ESN stored in the ESN memory and transmit only requests that contain matching ESNs, or substitutes the ESN from the ESN memory directly into the requests as they are being transmitted. In an alternative embodiment, the system formats the requests using the ESN from the ESN memory and message information supplied to it by the user.