As you likely know by now, Microsoft made an offer to buy Yahoo! today.
Given the machinations of such deals, as well as the regulatory hoops that must be jumped through, the completion of the deal could be a year or two away.
Granted that, it is far, far too early to figure out what the IP voice service offerings of a Yahoo! integrated within Microsoft might be.
Still, I see the maximization of both companies voice service assets as consisting at least in part as:
An integration of consumer-themed Yahoo! Voice and Yahoo! Messenger within a module of a future Microsoft Office Communications Server edition. While aimed at consumers, both of the Yahoo! products I mentioned could easily be adapted to the enterprise.
Click to call functionality in Microsoft Office apps. An icon in Excel, PowerPoint, Word, and Outlook could be used to, say, prompt a calling interface that would allow for real-time conferencing and collaboration with colleagues running the same apps.
For example, if I am in my Portland office ad wish to collaborate on a PowerPoint presentation with a colleague in Seattle, I'd click on a calling interface in my PPT, and would be taken either to my addy book or to a future variant of Yahoo! Voice. Then, I'd make the call and talk things over.
And that's just for starters.