An iPad with cellular capability activated has its own SIM card and unique telephone number and account but only the Telephone Company knows what that number is]. The iPad's hidden phone number is aliased to the iPhone's number. The user doesn't really see that separation (except on the monthly bill from the telco) but it is an interesting twist.
Just to clarify, the iPad's phone number is not a secret. The cellular provider will tell you what it is, and in my case, it shows up on the bill. Also displays on the iPad under Preferences>General>About>Cellular Data Number.
