[seanwilson]

> All the functions taking a string that might end up being a PhoneNumber need to be highly defensive

Yeah, I can't relate at all with not using a type for this after having to write gross defensive code a couple of times e.g. if it's not a phone number you've got to return undefined or throw an exception? The typed approach is shorter, cleaner, self-documenting, reduces bugs and makes refactoring easier.