An ITU standard (1968) for automatic calling and/or answering equipment on dial-up lines. It uses parallel circuits and is similar in function to RS-366 and Bell 801 autodialers used in the U.S. The answer tone defined in V.25 is the first thing heard when calling a modem. It serves a dual function of identifying the answering equipment as being a modem and also disabling the echo suppression and echo cancellation equipment in the network so that a full-duplex modem will operate properly.