Currently entered country code (mutable).
Default country code from server configuration (read-only).
Optionalextension?: stringCurrently entered phone extension (mutable, optional).
Label for the phone extension input (read-only).
Currently entered phone number (mutable).
Whether the phone extension input should be displayed (read-only).
Whether phone number format validation is enabled (read-only).
countryCode, phoneNumber, and optional extension are mutable values
passed to next(). defaultCountryCode, validatePhoneNumber,
showExtension, and extensionLabel are server-provided configuration.
Render an extension control when showExtension is true; the submitted
extension can still be omitted.
Phone number input collector.