code: public static Validator<CharSequence> phoneNumber(@NonNull final Context context) {
          return new PhoneNumberValidator(context, R.string.default_error_message);
      }
docs: Creates and returns a validator, which allows to validate texts to ensure, that they
  represent valid phone numbers. Phone numbers, which are only consisting of numbers are
  allowed as well as international phone numbers, e.g. +49 1624812382. Empty texts are also
  accepted.
  
  @param context
  The context, which should be used to retrieve the error message, as an instance of
  the class {@link Context}. The context may not be null
  @return The validator, which has been created, as an instance of the type {@link Validator}