Calling a stored procedure from RPG

Does anyone know how to call a SQL Stored procedure from RPG and pass in integer and receive the response back in the output variable?

We are having an issue where the input parameter doesn't appear to be mapped to the input parameter (RTID), and the CREDSTRING output variable is always blank.

We are able to successfully call the procedure using straight SQL. It just doesn't work from RPG.

Here is the SQL Stored procedure def:

CREATE PROCEDURE TESTLIB.GETCREDSTRING (
IN RTID INTEGER ,
OUT CREDSTRING VARCHAR(50) )
LANGUAGE SQL
SPECIFIC TESTLIB.GETCRDSP
NOT DETERMINISTIC
MODIFIES SQL DATA
CALLED ON NULL INPUT
SET OPTION ALWBLK = *ALLREAD ,
ALWCPYDTA = *OPTIMIZE ,
COMMIT = *NONE ,
DECRESULT = (31, 31, 00) ,
DFTRDBCOL = TESTLIB,
DYNDFTCOL = *NO ,
DYNUSRPRF = *USER ,
SRTSEQ = *HEX
BEGIN
....
...
END ;

One input parameter RTID (integer), and one output parameter (credstring)

Our RPG program is as follows:

CTL-OPT DATFMT(*USA) DEBUG(*YES) DFTACTGRP(*NO) ALWNULL(*USRCTL);

DCL-S InParm int(10);
DCL-S OutParm VARCHAR(50);
InParm = 0000600002;
EXEC SQL
call getcrdsp (:Inparm, :Outparm);
*INLR = *ON;
Return;