This Recommendation describes an objective single-ended method for predicting the subjective quality of 3.1 kHz (narrow-band) telephony applications. This Recommendation presents a high- level description of the method and advice on how to use it. An ANSI-C reference implementation, described in Annex A, is provided in separate files and forms an integral part of this Recommendation. A conformance testing procedure is also specified in Annex A to allow a user to validate that an alternative implementation of the model is correct. This ANSI-C reference implementation shall take precedence in case of conflicts between the high-level description as given in this Recommendation and the ANSI-C reference implementation. This Recommendation includes an electronic attachment containing an ANSI-C reference implementation and conformance testing data.