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In NGNs, handovers between different wireless access technologies provide seamless roaming during voice calls. The resulting speech quality depends on the audio bandwidth of the speech codecs used in the respective networks, as well as on degradations resulting from the handover, coding, and packet loss. We present the results of four listening experiments where speech quality is quantified as a function of network and codec characteristics, and compare them to estimations obtained from instrumental models. The results show when and under which circumstances a network handover and/or codec changeover should be scheduled in order to obtain better speech quality. This is important for the development of high-quality roaming strategies.