Im rather new to the iPad scene so this might already exist or at least being discussed, however I havent found any info...
This is not a question if you can call using your ipad carrier or Skype or any other service, instead I wonder why it wouldnt be possible to just get the iPhone INTERFACE up on the iPad via bluetooth.
I use the ipad in the car and if the app would exist to get the interface on the ipad screen and type the numbers and using the phonebook from there (but still using the carrier in the iPhone that are nearby in the car, connected via bluetooth)
Its possible to get the phone interface in many of the onboard displays and other gadgets (parrots for example). Why not the iPad? Instead of pressing the small buttons on the phone, you could do it on the iPad. See attached picture.