It's not possible to delete the phone app. So, before you go restoring your phone from a back up, look in all of the folders on all of the pages. If you still can't find it, you can reset your home screen layout. This will put all of the pre-installed apps back where they were when the phone was fresh out of the box. It will, unfortunately, get rid of all of your custom folders.
Settings>General>Reset>Reset Home Screen Layout
Best of luck.