This Recommendation describes a unified method required for calculating loudness, allowing comparison of narrow-band (300-3.4k Hz), wideband (100-8k Hz), super-wideband (50-14k Hz) and Fullband (10-20k Hz) telephony, for all types of terminals including handset, hands-free and conference terminals. The model described in this Recommendation is consistent when switching from one bandwidth to another and independent of the listening situation (handset, headset, hands-free …) with regards to producing a constant perceived loudness. Compered to loudness rating models, like the one presented in Recommendation ITU-T P.79, the present method predicts the absolute loudness, considers auditory masking and is applicable to a wide range of acoustic levels. This Recommendation incorporates a number of annexes that hold test vectors for validation of loudness model implementations as well as of the descriptions and results of the loudness experiments that form the basis for this Recommendation.