This is typically the "backdoor" number into Unity from the outside world. What I usually do is configure a CTI route point in CallManager which is a DID from the outside world. Forward to VM all checked mark.
Now, go into Unity, create a new CallHandler with this DID as the extension.
Setup the Caller Input for "*" to login or whatever you want.
I record the greeting to say "hello, you have the reach the voicemail system for company xyz, if you a voicemail mailbox on this please press star and enter mailbox ID and password"
Then I repeat the message a couple times and then hang up.
You might be able to get "fancier" by putting in the cell number or home phone in the subscribers profile. This way, when Unity see's the calling number, it will associate it with the subscriber, prompting for only the password,pin. But callhandler then has to go right to sub sign in from the outside world, no custom greeting.
Give it try. if you need more help, just post back.
Cheers
