Why are some apps hidden and how to unhide them For example, on my Galaxy Tab 7.7 Plus (SC-01E) running Android ICS, the Phone app APK is installed but does not show up in my app drawer. 

It will actually show up if it is called by a widget, but there is no launched icon for it anywhere. In the system settings it shows as running.

This tablet is not rooted but I did buy it off from Yahoo! auction.
Android has many apps that run either in background (such as Accuweather daemon) or only appear as widgets (for example TypoClock). Since these two apps don't need the user to open them from the launcher, they don't add an icon to it.

The same happens with Live Wallpapers. If you use a root explorer or another root app that will let you see all the installed apps, you will see that the list is higher than the apps that appear in the launcher.