An auxiliary dialing system for a mobile telephone of the type having a handset coupled with a radio transceiver by a data transmission line comprises a data process and control module having a microprocessor and data storage devices. The data process and control module includes a key pad for entering data into the data storage devices and a display for displaying data entered from the keyboard and retrieved from the data storage devices. The data process and control module is connected to an interface module which is in turn spliced into the data transmission line between the handset and the radio transceiver. The interface module insures that when a telephone number is transmitted from the data process and control module to the transceiver, the data is placed on the data transmission line in such a way that it does not interfere with or corrupt data being transmitted from the handset to the transceiver and vice versa in normal operation of the mobile telephone. Telephone number may thus be automatically dialed by the data process and control means by transmitting the numbers to the radio transceiver through the interface module so that the radio transceiver places the telephone call as if it had been dialed from the handset.