Without any other context? Nothing - it's just a type alias...

But the context this type of an alias should exist in is one where a string isn't turned into a PhoneNumber until you've validated it. All the functions taking a string that might end up being a PhoneNumber need to be highly defensive - but all the functions taking a PhoneNumber can lean on the assumptions that go into that type.

It's nice to have tight control over the string -> PhoneNumber parsing that guarantees all those assumptions are checked. Ideally that'd be done through domain based type restrictions, but it might just be code - either way, if you're diligent, you can stop being defensive in downstream functions.

Validation, readability, and prevention of accidentally passing in the wrong string (e.g., by misordering two strings arguments in a function).

An explicit type

What did you lose?