Problem getting sqlcode -10000 trying to call function to return VARCHAR2 variable

Trying to code an embedded OpenESQL call to a function in an Oracle database which returns a VARCHAR2 result.
Using Oracle ODBC driver 11.2.

They get sqlcode -10000 on the call but the same call seems to work for SQL Server database.

ORACLE stored procedure function call:

create or replace
function function1 return varchar2 as
begin
  return 'some value';
end function1;

COBOL:

       identification division.
       program-id. Program1.

       environment division.
       configuration section.

       data division.
       working-storage section.
           EXEC SQL INCLUDE SQLCA END-EXEC.
           EXEC SQL BEGIN DECLARE SECTION END-EXEC.
       01  WS-PARAMS.
           05 PARAM5.
              49 PARAM5-LEN PIC S9(4) COMP-5.
              49 PARAM5-ARR PIC X(20).
           EXEC SQL END DECLARE SECTION END-EXEC.
       procedure division.
       a.
           EXEC SQL CONNECT TO "TESTORA" END-EXEC.
          
           MOVE 20 TO PARAM5-LEN.
           MOVE SPACES TO PARAM5-ARR.
           EXEC SQL
              :PARAM5 = CALL FUNCTION1
           END-EXEC.
           DISPLAY "SQLCODE IS " SQLCODE.
                    
           EXEC SQL DISCONNECT END-EXEC.
          
           goback.

       end program Program1.

Convert the function call to a procedure with an output parameter as this works.

create or replace function function2(x output) as  
begin 
    x = 'some value'; 
end function2; 

           EXEC SQL 
              CALL PROC2(:PARAM5 out) 
           END-EXEC. 

Another alternative that doesn't require changing the server code is: