With the advent of multimedia traffic, Third Generation mobile systems were introduced, with the UMTS as the most promissor candidate. In consonance with the global trend of utilization of the IP technology in fixed networks, the UMTS developers decided to incorporate this protocol, which presents more facilities for the introduction of new services. Therefore, the UMTS architecture has two transmission modes: Circuit Switching and Packet Switching Modes. This Doctoral Dissertation initially describes the call setup processes (call flows) for those transmission modes. It follows a comparative evaluation of the efficiency aspect related to calls executed in both modes, considering the case of uplink transmission. In this process, several optimization techniques are introduced, aiming at the improvement of energy consumption in the air interface. The final objetive is to turn the Packet Switching Mode competitive with the Circuit Switching Mode, relative to the energy consumed for the transmission of voice calls. Though the treatment here presented refers to the UMTS-FDD technology, all the results are equally applicable to other similar systems, like the CDMA- 2000.