Convert Varchar To Datetime and Add Seconds
Let me know how this works for you.

CREATE TABLE #t (callstarttime VARCHAR(255), callduration INT);
INSERT #t ( callstarttime, callduration )
SELECT *
FROM (
VALUES('20180101215910', 120),('20180101215710', 220)
) AS x (callstarttime, callduration);


WITH munge AS (
SELECT *, 
    LEFT(callstarttime, 8) AS d,
    STUFF(STUFF(SUBSTRING(callstarttime, 9, LEN(callstarttime)) , 3, 0, ':'), 6, 0, ':') AS t
FROM #t AS t
)
SELECT *, 
       TRY_CONVERT(DATETIME, d + ' ' + t),
       DATEADD(SECOND, munge.callduration, TRY_CONVERT(DATETIME, d + ' ' + t))
FROM munge