BlackBerry Mobile Voice System is not a new offering for Research in Motion.  In fact, BlackBerry MVS, which integrates BlackBerry Enterprise Server with corporate PBX systems allowing you to make and receive calls via your BlackBerry utilizing your office number over Wi-Fi, has been around for over two years.

What is new is the new partnership between T-Mobile and RIM where T-Mobile will utilize BlackBerry MVS to offer a new service, “Wi-Fi Calling with Mobile Office“, to Enterprise customers.  Wi-Fi Calling with Mobile Office will be similar to T-Mobile@Home, but, for businesses.

Al Sacco from sums it all up here