MOS or Mean Opinion Score gives VoIP testing a number value as an indication of the perceived quality of  received voice after being transmitted and compressed using codecs.  This measurement is the result of underlying network attributes that act upon data flow and is useful in predicting call quality and is a good VoIP test tool in determining issues that can affect your VoIP quality and your conversations.

Using MOS scores to evaluate your VoIP service.

Testing the quality of VoIP has become easier and services have greatly improved over the last few years due to both the providers becoming more reliable and the ISPs offering better connections.  This advancement in the quality of services has helped increase the number of VoIP subscribers, but occasionally issues affecting voice quality do arise and being able to test your VoIP and identify these instances can be helpful in addressing them.  Having a metric to measure changes or degradation in the quality of the voice/VoIP connection after testing can help identify problems.  VoIP calls often are in the 3.5 to 4.2 MOS range.  The following chart can be used as a guide for VoIP MOS testing and a good comparison for voice quality.

Maximum for G.711 codec 4.4
Very satisfied 4.3-5.0
Satisfied 4.0-4.3
Some users satisfied 3.6-4.0
Many users dissatisfied 3.1-3.6
Nearly all users dissatisfied 2.6-3.1
Not recommended 1.0-2.6

MOS comparison values