parser(viewValue) {
    return parsePhoneNumber(viewValue, {
      regionCode: /** @type {RegionCode} */ (this.activeRegion),
    });
  }
@configure FormatMixin
@param {string} viewValue a phone number without (or with) country code, like '06 12345678'
@returns {string} a trimmed phone number with country code, like '+31612345678'