I'm not sure I'm following all of this description here... you're bouncing around a bit.
First, by default if we don't know who the calling number is on a direct call, the routing rules will send the caller to the active greeting for the Opening Greeting call handler created by setup - it's supposed to do this.
Presumably you want internal callers who are not subscribers who are accessing voice mail using the messages button to go to a different call handler than outside callers are being routed to? I'm just guessing, you didn't really state what it is you want the system to do here exactly.
Assuming that's the case, you can create a new routing rule that's sorted just above the opening greeting rule at the bottom that routes all calls that come from an internal extension (i.e. the calling number would be between 1000 and 9999 for instance if it's a 4 digit plan). This way callers who ARE subscribers are signed in as normal but other internal callers dialing in directly would get sent to whatever call handler you dictate in the routing rule.
I'm pretty confused by you're last sentence there... if you set the 0 key on a call handler up to "go to greeting for" (which is what I assume you mean) then it's _supposed_ to play the greeting. If you want it to dial the receptionist you'd use the "attempt transfer for" option and make sure the target call handler for the receiptionist is setup to ring the phone.
If this isn't what you were looking for, perhaps spend a little time and clearly outline what, exactly, you want calls to do when they enter your system and what criteria you'd like to route them on and how you'd like unity to behave for user input and we'll see if we can walk you through it.
