HI,
You need cisco unity with Voicemail option at the least to have your voicemail sent to your inbox and as for the cell phone forwarded to voicemail you need to do a call forward No answer or busy to your voice network's voicemai pilot number. ( Your service provider must support such call forward feature before this can be done)
The caller calling your cell phone would get a generic greeting promting for an extention or login.
To overide this please see this url for how this can be done. You may need to do some translation for incoming calls to the internal voicemail box and the follow the urlbelow.
http://www.cisco.com/en/US/partner/products/sw/voicesw/ps556/products_tech_note09186a00800dea82.shtml
Its a bit tricky but hope it helps
REgards
Tola.