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As broadband and high-speed IP networks spread, IP-telephony services have become a popular speech communication application over IP networks. Recently, the speech quality of IP-telephony services has become close to that of conventional PSTN services. To provide better speech quality to users, speech communication with wider bandwidth (e.g., 7 kHz) is one of the most promising applications. To ensure desirable quality, we should design the quality before services start and manage it while they are being provided. To do this, an effective means for estimating users perceptions of speech quality is indispensable. This paper describes a model for estimating the conversational quality of wideband IP-telephony services from physical characteristics of terminals and networks. The proposed model takes into account the quality enhancement effect achieved by widening speech bandwidth and has the advantage that it can evaluate the quality of both wideband and telephone-band speech on the same scale. Based on subjective conversational quality evaluation experiments, we show that the proposed model can accurately estimate the subjective quality for wideband speech as well as for telephone-band speech.