Members of a on-call team can be scheduled on the planboard, according to their 'on call' priority. This defines their on-call duty. Per individual team member, you can define
exceptions to the on-call duty, to specify that a person is not available a particular date / time and will be replaced by another team member for the time of absence. See also:
Scheduling on-call team duty and
Adding exceptions and replacements for on-call duty.