You can try to simulate with iPerf and very small UDP frames, but analytically it's pretty weak. Ixia IxChariot can simulate voice traffic between endpoints, using different codecs, etc and will report packet loss, jitter, etc ($$$$). Otherwise, Like Colin said, it's almost cheaper/free to use real voice clients (soft SIP phones on laptops against local freeware VoiP servers cost nothing, but you only have the voice quality and the UCC metrics from the controller to go by as far as analytics).