You have functions

    void callNumber(string phoneNumber);
    void associatePhoneNumber(string phoneNumber, Person person);
    Person lookupPerson(string phoneNumber);
    Provider getProvider(string phoneNumber);

I pass in "555;324+289G". Are you putting validation logic into all of those functions? You could have a validation function you write once and call in all of those functions, but why? Why not just parse the phone number into an already validated type and pass that around?

    PhoneNumber PhoneNumber(string phoneNumber);
    void callNumber(PhoneNumber phoneNumber);
    void associatePhoneNumber(PhoneNumber phoneNumber, Person person);
    Person lookupPerson(PhoneNumber phoneNumber);
    Provider getProvider(PhoneNumber phoneNumber);

Put all of the validation logic into the type conversion function. Now you only need to validate once from string to PhoneNumber, and you can safely assume it's valid everywhere else.

Can't pass a PhoneNumber to a function expecting an EmailAddress, for one, or mix up the order of arguments in a function that may otherwise just take two or more strings

That's going to be up to the business building the logic. Ideally those assumptions are clearly encoded in an easily readable manner but at the very least they should be captured somewhere code adjacent (even if it's just a comment and the block of logic to enforce those restraints).

If you are not checking that the phone number is 10 digits (or whatever the rules are for the phone number for your use case), it is absolutely pointless. But why would you not?

And parentheses. And spaces (that may, or may not, be trimmed). And all kind of unicode equivalent characters, that might have to be canonicalized. Why not treat it as a byte buffer anyway.