This paper provides an analytical model for the performance study of structured P2P overlays (based on DHTs) used for lookup service in IP telephony systems. The overlay created by supernodes, provides an infrastructure for the lookup service required before an actual voice communication is initiated. We model the P2P overlay as a queuing network. Our model is generalized and can be applied to evaluate any DHT algorithm performance. We extract useful observations and obtain closed form expressions for the session setup delay and capacity as a function of the number of participating supernodes. The analysis also attempts to answer the question of finding an optimum number of supernodes for minimum session setup delay. The generic analysis is applied in two specific cases of structured P2P, Ring and Chord DHT. Moreover, simulation based performance evaluation is also carried out to uncover the utility of the analysis