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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _serialize(self, value, attr, obj):
 """Format and validate the phone number user libphonenumber."""

        value = super(PhoneNumberField, self)._serialize(value, attr, obj)

        if value:
            value = self._format_phone_number(value, attr)

        return value