DELETE  MobileInfo

FROM    MobileInfo b
JOIN    (
    SELECT  MIN(a.CreatedOn) CreatedOn ,MobileNo    
    FROM    MobileInfo a
    GROUP   BY  a.MobileNo
) c ON
 c.MobileNo = b.MobileNo
AND c.CreatedOn <> b.CreatedOn
and DATEDIFF(MINUTE,c.CreatedOn, b.CreatedOn)<15