I can certainly see them move into the telecoms arena (bear in mind it's an industry worth an annual $800bn - who wouldn't want even 0.1% market share?).
I think the impetus to move is more about protecting their existing position in relation to IM market share - MS would not want to lose MSN users, and other IM products (Yahoo!, Skype and now Google) are edging into that market. PSTN breakout is one way of ensuring that you at least offer as much as the best of the competition.
Microsoft had a very interesting project on the burner a couple of years ago called Portrait:-
Nothing ever really happened with it.
I can see the technology in Portrait being wrapped up into MSN messenger, PSTN breakout provided by the Teleo purchase and the whole thing bundled with next generation Windows.
I think the key point of interest here, certainly mine anyway, is the way that this kind of technology (and it's obvious it's going to be a big thing) impacts the webmaster, and the WebmasterWorld member.
I can foresee a complete ressurrection of the "callto://" tag in web pages. We've seen similar already with online stores offering a type of "chat now" service based on a java text/instant messenger type script. Many online vendors find that kind of customer interaction very valuable. But being text based just makes it a little quirky.
Now look at what is happening with voice on the internet now, and how that is being bundled into "presence" applications such as instant messaging across the board by the big IM outfits. And then take a look at some user numbers (figures are a little dated now, but serve the purpose):-
MSN : 23.1m
Yahoo! : 19m
AIM : 31.9m
Skype : 15m (depending on who you believe)
.... and that makes nearly 90 million people out there potentially with VoIP on their desktop ready to be launched with a click of a callto:// tag.
Now, if you like to talk to your customer while he's surfing your website...