Simplex mode is trying to emulate real life conversation, where as only one side will talk at any time. In duplex mode testing, however, both sides will be talking at the same time, more a “double talk” voice testing scenario. It should be noted that duplex mode should never be used on analog device and in test setup that is sensitive to “double talk”. In either case, part of the original voice samples may be dropped because of they could be mis-interpreted as unwanted signals (echoes). This results in a degradation of voice quality, which does not represent the reality. You may use duplex mode in connections whereas there is an end to end transmission paths for each way of transmission (Send and Receive), AND there is no intermediate device between the two sides. Under this condition, the duplex mode will produce a more rigorous test result. If in doubt, use simplex mode