'On Call' enables you to set your phone to silent mode and still receive an audible alert if you miss a call that arrives between a time period that you specify.
This is particularly useful for workers with an on call period that starts during the night. For example you may work in a profession that requires you to be on call from 3am. When you go to bed in the evening you set your phone to silent mode and set 'On Call' to be active from 3am. Should any calls arrive before 3am the phone will remain silent however if a call comes in after 3am the phone will still obey silent mode but once the call rings out (or goes to message bank) an alert will sound to wake you up and notify you that you have just received a call/message which you can then action by checking the message and returning the call.