It's called Microsoft Office Communicator and we could see it this year. Worth a full read, its only a page or so.
MICROSOFT has developed a Skype-style free internet voice service for mobile phones that City analysts believe could wipe billions off the market value of operators such as Vodafone.
The service is included in a mobile version of Microsoft Office Communicator due to be released this year. It will take the form of a voice-over internet protocol (VoIP) application that allows Office users to make free voice calls over wi-fi enabled phones running Windows Mobile software. It uses the internet as a virtual phone network as well as accessing e-mail, PowerPoint and other Office applications. ...
Trying to downplay the havoc Microsoft Office Communicator Mobile will wreak on the mobile telecoms industry, Ballmer chose a topical Valentine’s Day theme for his announcement. “I love the mobile industry and I love our operator partners, and I want to have that message precede all we’re about to show,” Ballmer said in Barcelona. He went on to demonstrate how a mobile phone running Windows Mobile can be used to make a free voice call over the internet. Ballmer told the audience: “That was a VoIP call.”
But Ballmer’s announcement may be closer to a St Valentine’s Day massacre than a love letter for the mobile operators concerned.
We've attached a picture of Steve Ballmer consoling the telecom operators, "Well I love you, but it's just that I'm not in love with you..."
By the way, Wikipedia on Ballmer:
Steve Ballmer has been known to be very passionate in expressing his enthusiasm. Once his vocal cords required surgery after he screamed "Windows, Windows, Windows" continuously at a meeting in Japan in 1991. When Microsoft celebrated its 25th anniversary in 2000, Ballmer popped out of the anniversary cake to surprise the audience. His wild screaming and dancing on stage at an employees convention was caught on a widely-circulated video known as "Dance Monkeyboy." A few days later at a developers' conference, a sweat-soaked Ballmer repeatedly chanted "developers" at least 14 times in front of the bemused gathering.
Ballmer has also called Linux "communism" and the GNU General Public License "a cancer,"  as well as more recently claiming that the most common format of music on iPods is "stolen". He advocates digital rights management and has said that "DRM is the future."