One of the most significant differences between a cell phone and a PDA is that a PDA may be turned on and off, but a cell phone is normally on all the time. What important feature was compelled to be...

One of the most significant differences between a cell phone and a PDA is that a PDA may be turned on and off, but a cell phone is normally on all the time. What important feature was compelled to be incorporated in mobile operating systems as a result of this?