Voice-over-Internet Protocol (VoIP) technology provides the transmission of speech over packet-based networks. The transition from circuit-switched to packet-switched networks introduces two major quality impairments: packet loss and end-to-end delay. This thesis shows that the incorporation of packets that were damaged by bit errors reduces the effective packet loss rate, and thus improves the speech quality as perceived by the user. Moreover, this thesis addresses the impact of transmission delay on conversational interactivity and on the perceived speech quality. In order to study the structure and interactivity of conversations, the framework of Parametric Conversation Analysis (P-CA) is introduced and three metrics for conversational interactivity are defined. The investigation of five conversation scenarios based on subjective quality tests has shown that only highly structured scenarios result in high conversational interactivity. The speaker alternation rate has turned out to represent a simple and efficient metric for conversational interactivity. Regarding the two-way speech quality, it was found that echo-less end-to-end delay up to half a second does not cause impairment, even for highly interactive tasks.