You are responsible for getting your shift covered. If you offer up a shift and no one takes it then you are responsible for coming in to work.
If you are sick or experiencing extenuating circumstances then you must notify your supervisor right away if you will not be coming in. Calling in should not be a regular occurrence and any employee who calls in 3 or more times a semester will be written up and could lead to termination.
Failure to come in for your shift without notifying a supervisor will result in a no call-no show and may lead to termination of your employment.