This Recommendation provides specifications for voice service transmission measurement devices that provide measurements on an in-service and non-intrusive basis. These In-Service, Non-intrusive Measurement Devices (INMDs) are utilised primarily for the measurement of voice-grade parameters such as speech level, noise level, echo loss and speech echo path delay. INMDs may also be used to measure parameters associated with digital transmission systems, in both circuit switched and packet switched networks, that impact the performance of the voice-grade channels being transported. This Recommendation specifies interfaces, measurement ranges, and accuracy requirements for measuring voice-grade transmission parameters as well as descriptions of optional functions associated with these parameters.