If you are using the embedded VM, then you shouldn't have any problem burning a port... You probably have 20 or 30 anyway, use one from the middle of the ports, don't use the first or last VM port.
You are getting dialtone because you haven't recorded a greeting... Have you build your RAD Greetings set, assigned it to a port, and logged into the admin mailbox and recorded the RAD set greeting?
Otherwise, I guess you could use an Information mailbox... Build a dummy extension (or nametag hunt group, or whatever) and mailbox, reroute always to VM, set the mailbox type to Information only, and record the greeting there.
There are literally a dozen ways to do this, and it varies based on your VM type.