The text plays on the phrase 'calling in sick', which is commonly used when an employee informs their workplace that they cannot come to work due to illness. By substituting 'sick' with 'dead', the joke creates a humorous and exaggerated situation, as one cannot literally call in dead. This substitution creates a pun by using the unexpected word 'dead' in place of 'sick'.
I used up all my sick days , so I ' m calling in dead .