MOS is commonly used for assessing the performance of voice transmission systems, including VoIP (Voice over Internet Protocol) networks. 

• Subjective Testing Origin: Traditionally, MOS is obtained through subjective tests where a number of listeners rate the quality of speech samples on a scale. These ratings are then averaged to give the MOS.

• Rating Scale: The MOS scale typically ranges from 1 to 5, where 1 indicates poor quality and 5 indicates excellent quality. The scale assesses factors such as clarity, background noise, and overall user satisfaction with the call quality.

• Use in VoIP and Telecommunications: In VoIP and other digital communication systems, MOS is used to assess the impact of compression, network delays, packet loss, and other factors that can affect call quality.

• Objective MOS Measurements: Given the impracticality of conducting subjective tests in many situations, objective methods have been developed to estimate MOS. These methods use algorithms to simulate human perception of voice quality.

• Importance for Service Providers: MOS is a crucial metric for service providers to ensure and maintain a high level of customer satisfaction. It helps them in tweaking and optimizing network parameters to provide better voice quality.

Mean Opinion Score remains an important tool in the telecommunications industry, providing a quantifiable measure of voice quality and helping guide the optimization and improvement of voice communication systems.