SELECT
    *
    , CASE
        WHEN ISDATE(CalledDate) + ISDATE(CONCAT(SUBSTRING([CalledTime],1,2), ':', SUBSTRING([CalledTime], 3, 4))) = 2                   -- Test that both columns are valid dates
        THEN CAST([CalledDate] AS DATETIME) + CAST(CONVERT(TIME(0), CONCAT(SUBSTRING([CalledTime],1,2), ':', SUBSTRING([CalledTime], 3, 4))) AS DATETIME) -- create datetime column
        ELSE NULL
    END [CalledDateTime]
from cte