This ITU-T Recommendation provides audio performance requirements and associated testing for telephone band (300-3400 Hz) digital telephones. Requirements and test methods are specified for the major audio transmission parameters including Sending and Receiving Loudness Ratings, frequency response, noise, distortion, spurious signals, sidetone, echo path and delay. This ITU-T Recommendation is only applicable to digital telephones using encoding conforming to ITU-T Recommendations G.711 (64 kbit/s, PCM) and G.726 (32 kbit/s, ADPCM). Lower coding rates, for example ITU-T Recommendation G.728 (16 kbit/s, LD-CELP), are still under study.