Juan,

I'm going to get straight to the point here.  I believe you are going to have a very hard time finding a "free" voicemail system that integrates with CUCM or too many other PBX's for that matter.  There are some external services that front-end calls and such for SMB (small-medium business) companies but those typically still require a monthly or annual charge.  In those cases, the solution typically provides a front-end (main) number and an Auto Attendant with ability to forward out calls to employees (could be cell or home or whatever in terms of the number used) or simply leave a voicemail for a particular user.  I'm not saying it's impossible that something exists but you'll probably spend more time looking when you could just look at lower cost, viable solutions that meet your needs - for example: Cisco Unity Connection, Cisco Unity Express, and so forth.  Even 3rd-party solutions can usually integrate with Cisco and may be very cost effective.

Hailey

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