The BlackBerry Mobile Voice System (MVS) is a server-based product designed to sit between your existing BES and your IP-capable PBX, effectively turning your BlackBerry handheld device into a remote extension of the corporate telephone system: if you are out of the office, or just not at your desk, incoming calls to your desk DDI can be forwarded automatically to your mobile; and similarly any calls you make from your handheld can be routed out through the remote PBX so that calls appear as having originated from your office (and can be billed accordingly using the PBX’s least-cost routing capability [LCR]), and calls to internal extensions from your remote handheld are effectively free.

The MVS product is an adaptation of the Ascendant Voice Mobility Suite, a company that RIM acquired wholly a period of time ago. An overview of the solution is available on the BlackBerry web site:

http://na.blackberry.com/eng/services/blackberry_mvs/

The solution does not necessarily require that you have a BES deployed already, but if you are deploying BlackBerry MVS as a standalone server it ONLY provides PBX integration functionality. The MVS server role cannot be installed on any existing BES infrastructure. Therefore a separate server is required for the MVS role if you wish to provide Email, PIM, MDS (Mobile Data Service) as well as MVS functionality (and a corresponding Windows Server license).
MVS can only be deployed alongside BES 4.1.5 or later, and currently only Windows Server 2000 or 2003 I supported (not Server 2008).
For PBX integration, the phone system needs to be IP-capable, or an IP gateway can be installed between the MVS server and the analogue PBX.
The solution also requires an additional client Service Book to be installed on the BlackBerry handheld, which must be running version xxx (TBC) of the client operating system or later.
As with the BES software, a back-end database is required to store configuration information. For small-scale deployments the internal MSDE database can be used, but for larger deployments a separate Microsoft SQL Server installation is recommended.

Because all client-server communication is routed via the BES, sessions are encrypted and-to-end.

As you’d expect from a BlackBerry product, the solution allows the administrator to define and enforce policies controlling the usage of the handheld: restricting the numbers that can be dialled (long distance, international, etc), limiting the hours between which calls can be made, and all calls can be logged and/or recorded for compliance with regulatory or internal corporate requirements.
Users can be assigned a single telephone number that need not change regardless of the client device or mobile network operator used by the employee.
Users can also enjoy the same functionality that their deskphone allows them: calls can be transferred, put on hold, conferenced, etc. And the user need only access one voicemail, removing the need for the mobile operator’s voicemail service, and should a voicemail message be waiting in the user’s mailbox on the PBX, indication will be displayed on the handheld.
Outbound calls placed from the handheld will appear as originating from the corporate PBX (as effectively they are), and inbound calls to the user’s DDI will be delivered to the user’s handheld, including the calling party’s name and number (if they appear in the PBX’s address book directory).

The MVS solution works by 'marrying' up calls placed between the BlackBerry handheld and the PBX, and the external call between the PBX and the PSTN. For example, if you place a call from your handheld, you actually establish a call to the PBX, which then puts you on hold and places a call to the external number to be contacted. Once that number is answered, the two call 'legs' are connected. A similar process occurs for incoming calls.

BES MVS is slated for release in Q1 2009.