Mean Opinion Score (MOS) is commonly used to rate phone service speech quality, expressed on a scale from 1 to 5, where 5 is best. A score of 4 is normally considered "Toll Quality" for calls placed over PSTN/TDM networks. MOS is a function of many factors, including the type of network and codecs used, wiring and premises equipment, and even the handset used to place the call.
MOS was originally determined using subjective listening tests, where a panel of trained experts judged recorded speech samples to assign an averaged score. Test equipment calculates MOS using sophisticated algorithms that are designed to closely approximate the results of subjective listening tests.
MOS is an overall indication of speech quality, aggregating dozens of speech impairments into a single score. Because it is a general metric reflecting many factors, the MOS score should not be used exclusively to characterize speech quality. The effects of echo, delay, call volume and clipping may not significantly influence MOS scores calculated by some algorithms, while actual callers may be quite irritated by their presence, especially if they affect a significant portion of the call.
A thorough speech quality assessment is best performed by reviewing primary quality measurements such as noise, distortion, echo, and delay, along with MOS from a number of algorithms, if possible. Knowing the strengths and weaknesses of the MOS algorithms used is critical to accurately interpret the resulting scores.