If a code is transmitted at a first transmission rate, a base station modulates the entirety of the code, namely a preamble, a synchronizing signal and a call-data, by two values and transmits them at the first transmission rate. If the code is transmitted at a second transmission rate, which is double the first transmission rate, the base station modulates the preamble and the logically inverted synchronizing signal (obtained by inverting the "1", "0" logic of the original synchronizing signal) by the two values, transmits them at the first transmission rate, modulates the call-data by four values and transmits it at the second transmission rate. As a result, it suffices to provide the pager with a single common preamble detector and a single common synchronizing detector, irrespective of the transmission rate. Moreover, by detecting the synchronizing signal or its inverted version, it is possible to determine whether the call-data has been modulated by two values or four values, thereby assuring that the call-data following the synchronizing signal can be modulated correctly.