Retrieves the preferred country calling code for region_code, e.
g. 358 for "fi" or 1 for "en_US@UTF-8".
If null is passed for region_code the default region as returned by
phone_number_get_default_region is used.
| region_code |
a two-letter country code, a locale name, or null |
|
a valid country calling code, or zero if an unknown region code was passed. |