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Since the apparition of Voice over IP (VoIP), many standards (mainly signaling protocols and codecs) have arisen with the aim of enabling voice calls through data networks. This paper describes a comparative study to evaluate the performance of a representative set of these standards when transmitting voice streams in a packet network. Asterisk PBX software is employed to define a testbed that allows to monitor diverse performance parameters in actual VoIP connections after the call establishment phase. Performed tests include the use of SIP and IAX (with and without trunking) signaling protocols as well as four different codecs. The obtained Results (expressed in terms of processor utilization, memory usage and network bandwidth consumption) show the clear benefits of employing IAX with trunking.