As the channels of any international telephone circuit should be suitable for voice-frequency telegraphy, theaccuracy of the virtual carrier frequencies should be such that the difference between an audio-frequency applied to one end of the circuit and the frequency received at the other end should not exceed 2 Hz, even when there are intermediate modulating and demodulating processes.
ITU-T G.225 FRENCH-1993 Referenced Document
ITU-T G.103 Hypothetical Reference Connections Study Group 12*， 1998-04-19 Update
ITU-T M.540 Routine Maintenance of Carrier and Pilot Generating Equipment - General Maintenance Principles - Maintenance of International Transmission Systems and Telephone Circuits Study Group IV; 4 pp