If a user can be called on multiple numbers, is there a way to know which number the remote caller dialed? Something like "getLocalNumber"?

There are so many ways to manipulate called number, in so many places - the answer is no. In a few rare cases you can get one of the vector directory numbers (there can be a sequence of VDNs used - you only get one of the numbers) in lieu of some other version of the extension that is ringing - that occurs in the context of contact center calls and it is my impression that is not what you are working on.