A method for use of IN-service Non-intrusive measuring Device (INMD) for proving Service Level Agreements (SLAs) and pro-active maintenance in IP telephony networks is described. The INMD is currently used only in connection oriented TDM telephony networks and therefore cannot be used directly in the connectionless-featured IP telephony networks. The method for use of INMD to measure voice transmission quality in IP telephony networks include a set of pre-processes for processing IP voice datagrams, and converting the pre-processed datagrams to a format suitable for INMD before the INMD evaluates data in the datagrams for voice GOS. Various tests and co-processes are also used to determine network performance parameters. An IP telephony measurement apparatus for practicing the method is also described. A method for calibrating the device and algorithms is also provided to ensure a high level of confidence in the measurement accuracy.