Something

        SELECT
                SUM(c.billsec/60) AS ttime,
                COUNT(c.uniqueid) AS tcalls, 
                CONCAT(e.FIRST_NAME, ' ', e.SECOND_NAME) AS emp,
                IF( c.userfield = '', c.src, c.userfield) AS ext
        FROM
                cdr c
        LEFT JOIN
                extentions x ON (c.src=x.channel OR c.userfield=x.extention)
        LEFT JOIN
                employee e ON (e.extention=c.src OR e.extention=c.userfield)
        WHERE
                c.lastapp = ?
        AND  
                c.disposition = ?
        AND
                c.accountcode LIKE ?
        AND
                ((c.src BETWEEN ? AND ? ) OR (c.userfield BETWEEN ? AND ?))
        AND
                (c.calldate BETWEEN ? AND ?)
        GROUP BY IF( c.userfield = '', c.src, c.userfield )