VoIP quality must be measured to ensure appropriate customer satisfaction. Recent state-of-the-art technology is significantly advanced by VoIP quality monitoring and testing systems based on the E-Model, an ITU G.708 standard scheme whose key contribution is that it passively measures each call more accurately without having to generate costly and intrusive test calls. Administrators may use systems that are E-Model based to proactively measure the behavior of their VoIP network, baseline the performance, and understanding the human perception of the call quality, thus predict the MOS score. Passive techniques are best for bringing the problems to light, providing accurate information that enables a rapid fix.