According to the invention, a mobile telephone includes a vibrating alarm (T) arranged in a module (4, VIBMOD) which is detachably connectable to the telephone. The module may be a changeable battery pack (4). The operating mode of the alarm device (T) and the alarm signal is controlled from the operating menu of the telephone. Operation of the vibrating alarm (T) is enabled or disabled through detection means and software in the telephone depending on whether the module is connected to the telephone or e.g. when the telephone is connected to a car kit or a charger.