Just like what they did with British Telecom (BT) they are doing that with carriers, rather than creating their own phone company using private labeled services from say, Level3.
Why? Because Yahoo knows what they know, and are smart enough to know what they don't. Becoming a telco is more than just offering termination. Plus, they don't compete by offering the ability to work with whichever carriers want to join in, and give the consumer or business user a choice.
Yahoo keeps making smart moves. This is just one more.