Calling a procedure in a service program that returns a char(6) value

Hi everyone, I'm stuck with what I thought was a small problem but can't
seem to resolve.

From a Java class running on a Windows server, I need to:

1. connect to a certain IBM i power server; (done)
2. set a specific library list with a given custom command; (done)
3. call a service program procedure that returns a char(6) value. (And here
I'm stuck.)
Despite several attempts, the procedure, assuming it's actually executed,
doesn't produce any results.
This is the RPG prototype of the procedure:
dcl-pr RtvSupplierCode Char(6) EXTPROC('RtvSupplierCode');
codMec zoned(7:0) const options(*omit:*nopass);
vatCode char(16) const options(*omit:*nopass);
idColCode packed(13:0) const options(*omit:*nopass);
end-pr;

The returned char(6) field may also be empty.
The three parameters are alternative; those not passed must be marked with
the *OMIT placeholder. The possible values for the test are:
codMec=some numbers or vatCode=some numbers but as a string or
idColCode=again some numbers

Normally, the call in rpgile would be
supplierCode = RtvSupplierCode(1236547 : *omit : *omit);
supplierCode = RtvSupplierCode (*omit : '123456789012345' : *omit);
supplierCode = RtvSupplierCode (*omit : *omit : 12345);

Now on my class It just doesn't seem to work.

Has anyone ever used this call?
Does anyone have any working code examples I can try?
Thanks in advance for your attention.