VinceWhirlwind... I think what the user is getting at is using the "To reach this person's cell phone, press C the 2 key" in the embedded voicemail, many users like this rather than twinning so they don't get every call to their cell phone.
This can get a little complicated, but if you look in the online help in the embedded voicemail section, it spells out the basics pretty clearly. You will need to change the COR of the VM ports so they can dial out, meaning your ARS and toll control setup must be pretty solid. We setup a COR that only allows local 7 numbers and 1701, 1605, 1406, and 1218 (the neighboring area codes to our area) and allow the VM ports to only call those numbers and it handles 99% of situations.
It can be done with system speed dials, but that kind of defeats the purpose of the user being able to edit those settings. It cannot be done with user speed dials (the voicemail ports are what is actually dialing the numbers).