This paper aims to introduce the relationships between some Voice over IP Key Performance Indicators (KPIs) and to address them through the background of possible network or platform problems, resulting in KPIs degradation. We present extensions of a generic Quality of Service (QoS) monitoring strategy by describing a variety of correlations between the QoS metrics. For echo correlation for instance, background root causes are described, which can be useful in terms of troubleshooting process triggered after KPI degradations. Metrics correlation with potential problems description generally speed up troubleshooting process and helps to explain the impact of QoS related KPIs degradation. The correlation can be used to infer the causal relations between underlying faults.