The issue here is that CUE won't necessarily understand what you are asking it to do.
When someone dials an extn on the old TDM/PBX and it gets call forwarded to CUE, CUE needs to be passed the extension number that was originally dialled *in the correct format* so that it can match the correct mailbox up and play the right greeting.
Each PBX does this in an different, and often proprietory, way. It's this what Paolo (Paolo, please correct me if I'm wrong) is saying you need to understand.
I think it's unlikley that CUE would natively be able to do this (although I may be proved wrong).
You might want however to take a look at the Cisco PIMG (aka Dialogic) gateways. They are designed for exactly this kind of thing. I recently managed to get working a very old Index PBX (It's older than me) to use Unity voicemail by using a PIMG in the middle. Basically the PIMG connects to the PBX, and then routes into the voicemail system over IP performing the necessary translations - i.e. looking at what the PBX is trying to do / signalling, and then translating it into something that Unity understood. There was a *lot* of work that went on understanding how the PBX was signalling and then writing rules so that Unity was passed the correct information. This was Unity Voicemail, not CUE, so I'm not 100% certain that you could use PIMG for this, although PIMG does support SIP, so you may be in luck.
The tracing on the PIMG is pretty good, so you can see what DTMF digits are being presented by the PBX, and then write your own rules to route the call into the voicemail system in the correct way. There's a few examples available on CCO as well. TAC are also pretty good at supporting it - I had a hugely helpful TAC engineer help me configure it.
HTH. Barry
