The measurement methods adopted for measuring on operator telephone systems (OTS) which comprise a headset, feeding circuit and subscriber's line (the same principles can be applied to any system that uses a headset), conform to the methods described in Recommendation P.64 with the following exceptions.
ITU-T P.38 FRENCH-1993 history
1993ITU-T P.38 FRENCH-1993 Transmission Characteristics of Operator Telephone Systems (OTS)