The following should give you what you're looking for.

IF OBJECT_ID('tempdb..#temp') IS NOT NULL

DROP TABLE #temp;

CREATE TABLE #temp (

TN CHAR(4),

CallDate DATE

)

INSERT #temp (TN, CallDate) VALUES

('1111','2014-01-01'),

('1111','2014-02-01'),

('1111','2014-03-01'),

('1111','2014-06-01'),

('1111','2014-07-01'),

('1111','2014-10-01'),

('1111','2014-11-01'),

('1111','2014-12-01'),

('1111','2015-01-01'),

('1111','2015-02-01'),

('1111','2015-03-01'),

('1111','2015-04-01'),

('1111','2015-08-01'),

('1111','2015-09-01'),

('1111','2015-10-01'),

('1111','2015-11-01'),

('1111','2015-12-01');

SELECT

t1.TN,

t1.CallDate,

cc.CallCount

FROM

#temp t1

CROSS APPLY (

SELECT COUNT(*) AS CallCount

FROM #temp t2

WHERE t1.TN = t2.TN

AND t2.CallDate >= DATEADD(dd, -180, t1.CallDate)

AND t2.CallDate <= t1.CallDate

) cc

Results...

TN CallDate CallCount

---- ---------- -----------

1111 2014-01-01 1

1111 2014-02-01 2

1111 2014-03-01 3

1111 2014-06-01 4

1111 2014-07-01 4

1111 2014-10-01 3

1111 2014-11-01 4

1111 2014-12-01 4

1111 2015-01-01 4

1111 2015-02-01 5

1111 2015-03-01 6

1111 2015-04-01 6

1111 2015-08-01 3

1111 2015-09-01 3

1111 2015-10-01 3

1111 2015-11-01 4