I can't see MCI and MSN as a team, but apparently the deal is brewing, according to Overdo's Land of Nothingness (props for the name):

The new VOIP service from Microsoft, Windows Live Call, will only be included with Windows Live Messenger. Using this application and your Passport, you can quickly and easily call your Messenger buddies from your computer. Windows Live Call will also allow a test call during signup, so you can check voice quality and your computer configuration. Although the service is limited in some ways (e.g. 911 calls and calls to toll free numbers) you can participate in a conference call even though you cannot initiate one.

Rates are dependent on the country you are calling, with the VOIP service being provided by the communications company MCI.

More info on the Live Messenger Beta here, but no mention of MCI. This one hangs solely on the source at Overdo, though I feel pretty confident that it's probably legit.

Anyone know anything else on this?