I recently posted a feature article on LoveMyTool that deals with the inconsistency in Mean Opinion Scores (MOS) as reported by packet-based VoIP analysis tools. MOS is a rating system for voice transmission quality as determined by a group of human listeners. Computer generated versions attempt to emulate how such a group would rate the call quality.
Speaking from experience, some analyzers are far more accurate than others. The bottom line is that no one vendor uses the exact same algorithm to compute MOS unless they OEM the code from the same source. When selecting a tool, insist on test data to back up vendor claims; typically real call data under different scenarios validated against human listeners conducted by a legitimate third party tester.
I highly encourage you to read the full article.