code: def normalize(self) -> pd.DataFrame:
      self.phone_to_int()
      if self.country_column_name is not None:
          self.df = self.df.apply(self.add_prefix, axis=1)
      return self.df[self.phone_column_name].astype("Int64")
  
file_path: upgini/src/upgini/normalizer/phone_normalizer.py
imports: from typing import Optional
  import pandas
  from pandas.api.types import is_float_dtype, is_int64_dtype, is_string_dtype
  from upgini.errors import ValidationError
label_desc: Use image node_id 2 for calling the PhoneNormalizer obj's underlying member method code with example usage: obj.normalize() and returns: self