Check/untaint phone numbers. May be null. If there is an error, returns
the error, otherwise returns false.
-Takes an optional two-letter ISO country code; without it or with unsupported
-countries, ut_phonen simply calls ut_alphan.
+Takes an optional two-letter ISO 3166-1 alpha-2 country code; without
+it or with unsupported countries, ut_phonen simply calls ut_alphan.
=cut