asterisk users - Oct 2010 - meetme don't play conf-invalid if room does not exist

Has anyone a solution for me

- with "Meetme(,Ms)"        asterisk plays "conf-invalid" if
a room not exist
- with "Meetme(123,Ms)" asterisk plays not "conf-invalid" if
the room not exist and asterisk hangup

I am happy about any proposal.

Thanks
Daniel