Recommendation ITU-T P.310 provides audio performance requirements and associated testing methods for narrow-band (300-3400 Hz) digital handset and headset telephones.Requirements and test methods are specified for the major audio transmission parameters including sending and receiving frequency response,loudness rating,noise,distortion,out-of-band signals,linearity,sidetone,echo and delay.This Recommendation is only applicable to digital telephones using encoding conforming to Recommendations ITU-T G.711 (64 kbit/s, PCM) and ITU-T G.726 (32 kbit/s,ADPCM).IP terminals and wireless headsets are not covered in this recommendation.