public function add_sortable_column( array $sortable_columns ) {

		$sortable_columns[ MortgagePlatform::$prefix . 'lead_phone' ]            = array(
			'sort_' . MortgagePlatform::$prefix . 'phone',
			false,
		); // false = asc (default), true  = desc
		$sortable_columns[ MortgagePlatform::$prefix . 'lead_email' ]            = array(
			'sort_' . MortgagePlatform::$prefix . 'email',
			false,
		);
		$sortable_columns[ MortgagePlatform::$prefix . 'lead_zipcode' ]          = array(
			'sort_' . MortgagePlatform::$prefix . 'zipcode',
			false,
		);
		$sortable_columns[ MortgagePlatform::$prefix . 'lead_yearofbirth' ]      = array(
			'sort_' . MortgagePlatform::$prefix . 'yearofbirth',
			false,
		);
		$sortable_columns[ MortgagePlatform::$prefix . 'lead_investmentamount' ] = array(
			'sort_' . MortgagePlatform::$prefix . 'investmentamount',
			false,
		);

		return $sortable_columns;
	}