  The first astrophysical results of the VLTI focal instrument AMBER have shown
the importance of the differential and closure phase measures, which are
supposed to be much less sensitive to atmospheric and instrumental biases than
the absolute visibility. However there are artifacts limiting the accuracy of
these measures which can be substantially overcome by a specific calibration
technique called Beam Commutation. This paper reports the observed accuracies
on AMBER/VLTI phases in different modes, discusses some of the instrumental
biases and shows the accuracy gain provided by Beam Commutation on the
Differential Phase as well as on the Closure Phase.
