create table phoneno
(
pno varchar2(20)
);
insert into phoneno
select '13766666021' from dual
union
select '13760666601' from dual
union 
select '13766606660' from dual
;
commit;
select pno,
       substr(pno,instr(pno,'6666',1)),
       substr(substr(pno,instr(pno,'6666',1)),1,5),
       case when substr(substr(substr(pno,instr(pno,'6666',1)),1,5),1,4) = '6666'
                 and substr(substr(substr(pno,instr(pno,'6666',1)),1,5),5,1) <> '6'
            then 'YES'
            else 'NO' end
  from phoneno;