I have debugged this following procedure call using this procedure:

CREATE PROCEDURE ADD_NUMBERS (NUM1 INTEGER, NUM2 INTEGER) RETURNS (RES
INTEGER)
AS BEGIN
RES = NUM1 + NUM2;
END

And this CallableStatement:
{CALL ADD_NUMBERS(?, ?, ?)}

which I prepare and then call CallableStatement.getParameterMetaData()

This fails with:
org.firebirdsql.jdbc.FBSQLException: GDS Exception. 335544569. Dynamic
SQL Error
parameter mismatch for procedure ADD_NUMBERS

When debugging, I can tell that
{CALL ADD_NUMBERS(?, ?, ?)}
gets converted to
EXECUTE PROCEDURE ADD_NUMBERS(?, ?, ?)

I'm guessing the conversion to native procedure call is the problem?

Regards,
Nils