Conversion failed when converting date and/or time from character string - sql-server

Question

What I am doing is using own function, which is returning string, now I want this column for ordering, and while using it for ordering it is not ordering properly because its a string, when I tried to convert as datetime it causes an error.
Any help ?
Thanks a lot.
SELECT
b.CallId,
CONVERT(VARCHAR(25), b.ETADate, 103) as'ETADate',
dbo.getCallEntrySerialNoLastTranDateTime(b.CallEntrySerialNumbersId) AS 'closingDateTime'
FROM
CallEntry AS a, CallEntrySerialNumbers AS b
WHERE
a.ASPId = 2
AND a.CompanyId = 3
AND a.CallId= b.CallId
AND a.ProdCompanyId = 1
AND b.CallCaseId IS NOT NULL
AND b.CallCaseId NOT LIKE 'NA'
AND ProdCategoryid = 1
AND a.CallDateTime > dateadd(day, -30, getdate())
AND b.StatusId = 2
ORDER BY
dbo.getCallEntrySerialNoLastTranDateTime(b.CallEntrySerialNumbersId) ASC
// what I tried and causes above error is
ORDER BY
CONVERT(DATE, dbo.getCallEntrySerialNoLastTranDateTime(b.CallEntrySerialNumbersId)) asc
EDIT-added function
ALTER FUNCTION [dbo].[ASP_getCallEntrySerialNoLastTranDateTime]
(
#CallEntrySerialNumbersId bigint
)
RETURNS varchar(500)
AS
BEGIN
DECLARE #ReturnVal NVARCHAR(500);
begin
select #ReturnVal = ( select Top 1 CONCAT(CONVERT(VARCHAR(10),CallTranEndDateTime,103),' ',CONVERT(VARCHAR(10),CallTranEndDateTime,108)) from calltransactions
where CallEntrySerialNumbersId = #CallEntrySerialNumbersId AND CallTranTypeId = 3
order by CallTranId desc)
end;
RETURN #ReturnVal;
END

Answer 1

Try this one -
SET DATEFORMAT dmy
SELECT *
FROM (
SELECT
b.CallId
, ETADate = CONVERT(VARCHAR(25), b.ETADate, 103)
, closingDateTime = dbo.getCallEntrySerialNoLastTranDateTime(b.CallEntrySerialNumbersId)
FROM dbo.CallEntry a
JOIN dbo.CallEntrySerialNumbers b ON a.CallId = b.CallId
WHERE a.ASPId = 2
AND a.CompanyId = 3
AND a.ProdCompanyId = 1
AND ISNULL(b.CallCaseId, '') NOT LIKE 'NA'
AND ProdCategoryid = 1
AND a.CallDateTime > DATEADD(day, -30, GETDATE())
AND b.StatusId = 2
) d
ORDER BY CAST(d.closingDateTime AS DATE)
ALTER FUNCTION [dbo].[ASP_getCallEntrySerialNoLastTranDateTime]
(
#CallEntrySerialNumbersId BIGINT
)
RETURNS VARCHAR(30)
AS BEGIN
DECLARE #ReturnVal VARCHAR(30)
SELECT #ReturnVal = (
SELECT TOP 1 CONVERT(VARCHAR(10), CallTranEndDateTime, 103) + ' ' + CONVERT(VARCHAR(10), CallTranEndDateTime, 108)
FROM dbo.CallTransactions
WHERE CallEntrySerialNumbersId = #CallEntrySerialNumbersId
AND CallTranTypeId = 3
ORDER BY CallTranId DESC
)
RETURN #ReturnVal
END