I agree that on-call is necessary in certain professions (e.g, doctors). I also agree that if an employee is willing to do on-call and they are compensated accordingly, then the practice is still ethical.
However, to call someone to do work outside work hours is unreasonable. On-call is considered work time, so I am expecting to be contacted during that time. However, if I'm not on-call, then it is not time for me to work, and I shouldn't be contacted by my company and feel pressured to answer the call.