Try with this data and see what happens..

drop table mastertab

CREATE table mastertab(

op CHAR(2),

msisdn VARCHAR (12),

imei VARCHAR (20)

);

INSERT INTO mastertab

(op,msisdn,imei)

SELECT 'aa','0191','111222333' UNION ALL

--SELECT 'aa','0191','111222444' UNION ALL

SELECT 'aa','0192','111222333' UNION ALL

--SELECT 'aa','0192','111222444' UNION ALL

SELECT 'aa','0192','111222555' UNION ALL

--SELECT 'aa','0193','111222333' UNION ALL

SELECT 'aa','0193','111222334' UNION ALL --added

SELECT 'aa','0193','111222335' UNION ALL --added

SELECT 'aa','0193','111222336' UNION ALL --added

SELECT 'aa','0193','111222337' UNION ALL --added

SELECT 'aa','0193','111222338' UNION ALL --added

SELECT 'bb','0171','222222333' UNION ALL

SELECT 'bb','0171','222222444' UNION ALL

SELECT 'bb','0172','222222444' UNION ALL

SELECT 'cc','0152','333222444'

;WITH CTE AS

(

select op,imei, DENSE_RANK() over (order by op,imei) rankno

FROM mastertab

)

select c.op, COUNT(distinct rankno) - 1 cnt

FROM CTE c

GROUP BY c.op