The invention relates to a method of setting up a call between a central module (33) and a remote module (37) of the type comprising at least one first transmission channel and one second transmission channel according to distinct primary and secondary bit rates respectively. According to the invention, the method comprises the steps setting up a call by default on the first channel; listening and searching for a signal indicating a secondary bit-rate requirement, assuming a detection of the signal indicating a secondary bit-rate requirement, a phase for the switching of a bit rate from the first primary bit-rate transmission channel to the secondary bit-rate transmission channel, associated with a step for the masking, during a predetermined masking period, of an acknowledgement signal sent during the default setting-up phase. The invention also concerns a corresponding switching method, transmission network section and transmission network.