Abstract: A device for plugging into the data port of a mobile telephone, for enabling the mobile telephone to execute user applications; and a method for developing such devices and programming the applications therein. Application plugs may be inserted into the data port of the mobile telephone by the user for executing applications, without modifying any hardware or software of the mobile telephone itself. An exemplary telephonic application is presented, which monitors outgoing calls on the mobile telephone, and automatically reroutes long-distance calls through a calling-card or call-back service to obtain lower-cost calling rates. This application is transparent to the user and allows the user to place calls using the mobile telephone's phone book, speed-dialing, or redial features, and without having to enter local access numbers, calling-card account numbers, or other service access codes. Other applications, including non-telephonic applications, can be pre-programmed or downloaded by the user.