The rising of IP telephony demands an efficient tool for QoS prediction and measurement in order to allow the planning and designing of the underlying networks to meet quality requirements. Among the several tools available for the traditional circuit switched telephony networks, the E-model can be adapted to modern VoIP networks. This paper discusses this adaptation in a corporate WAN scenario in which PC users can communicate with each other. Impairments related to delay and terminals are analised to derive the proper formul?to be included in the E-model. Impact of other features of the IP networks and terminals in existing factors of the E-model have also been addressed.