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By default, you have to assume that they did not work the shift. They were on-call but did not get called into work. You do this by setting the start date worked and end date worked to the same value as the start date scheduled. Edit the 'As-worked' start and end time so that the 'As-worked' hours are equal to 0.
As for the process:
Note: If a shift is assigned in the revision screen using the provider selector, then the 'As-Worked' hours should be automatically set to the default values (and the time range will not be displayed).