An ISO 3166-1 two letter country code. If the number is passed in an international format (e.g. +44 117 496 0123), then the region code is not needed, and can be null. Failing that, the library will use the region code to work out the phone number based on rules loaded for that region.
+44 117 496 0123
An IETF language tag (defaults to en)
