Battery reserve for emergency calling

A useful feature for cellphones to have is to keep a backup reserve of energy that can only be used for emergency calls. In situations where this feature will be used, the phone will be off (as it is out of batteries), thus activating this feature will require some sort of hardware button. Thus, the button must be hidden away carefully enough that emergency calls are not accidentally made. For example, a possibility would be a little button under the battery cover.