Maintaining good Quality-of-Experience (QoE) is crucial for Voice-over-IP (VoIP) applications, particularly those operating across the public Internet. Accurate online estimation of QoE as perceived by end users allows VoIP applications take steps to improve QoE when it falls below acceptable levels. ITU-T recommendation G.107 introduced the E-model, which provides a means to assess QoE levels for two-party VoIP sessions. In this paper we provide an analysis of the accuracy of the E-model for multi-party VoIP sessions when all audio is processed by a centralised focus node. We analyse the impact of what we term the 'Focus Transcoding Effect (FTE),' the 'Focus Forwarding Effect (FFE),' and the number of end-points participating in the session. Through comparison to QoE metrics produced by the offline PESQ method for three common audio codecs, we show that the standard E-model does not provide accurate QoE assessment for multi-party VoIP sessions. We then introduce an improved Emodel for these codecs for multi-party VoIP sessions. We describe the implementation of the improved E-model in a QoE monitoring application, showing that it produces results similar to actual PESQ scores. © 2013 IEEE.