Someone just brought an iPhone to me which was missing an icon for the phone on the home screen! Now obviously this reduces the functionality of the device as a phone somewhat!
Turns out there’s an easy fix though:
Go into Settings, General, then “Reset Home Screen Layout”
You should now have a working phone icon again!
Update: Several reports suggest checking Settings, General, Restrictions as this may also disable the phone icon. More details on restrictions here:
http://support.apple.com/kb/HT4213
Update 2: Another tip reported – double tap the home button and look for the icon in the tray at the bottom of the screen. You can then drag it back from there.