Don't just give up lol.

Really I am just asking you to play around with H2 at the DB/driver level and figure out how this works so that I can implement that in Hibernate. If I had to guess I'd assume H2 expects something like:

CallableStatement cs = conn.prepareCall( "{CALL STRING_IN(?)}" );
cs.setString( "test" );
boolean isResultSet = cs.execute();
// play around with isResultSet / cs.getMoreResults()
ResultSet rs = cs.getResultSet();