The MOS and R-Factor do not tell the whole story about the quality of a particular VoIP system. These measurements are merely suggested baseline points for expected call quality in a VoIP deployment. In order to get an idea on the exact correlation between the numerical measures and the actual quality of the call, one has to listen to various conversations and give their judgment on the acceptable quality.
The ability to play back actual conversations is essential for successful VoIP deployments. A network analyzer provides the means to record and play back calls to assess voice quality by simply listening to the voice, as shown on the screen shot.