Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for evaluating the quality of a communication session. One of the methods includes identifying, by a communication system, a communication session between one or more users of the communication system, wherein, during the communication session, session data is routed between a first communications device of a first user of the communication system and one or more other communications devices along a communication path; obtaining, from each of a plurality of communication nodes along the communication path, quality data relating to a quality of the communication session at the communication node; generating, using the quality data, a model input to a quality score machine learning model; and providing the model input as input to the quality score machine learning model to generate the estimated quality score for at least the portion of the communication session.