preprocessor(viewValue, { currentCaretIndex, prevViewValue }) {
    return liveFormatPhoneNumber(viewValue, {
      regionCode: /** @type {RegionCode} */ (this.activeRegion),
      formatStrategy: this.formatStrategy,
      formatCountryCodeStyle: this.formatCountryCodeStyle,
      currentCaretIndex,
      prevViewValue,
    });
  }
@configure FormatMixin
@param {string} viewValue
@param {object} options
@param {string} options.prevViewValue
@param {number} options.currentCaretIndex
@returns {{ viewValue: string; caretIndex: number; } | undefined }