Check out this Article from Cnet News:
Twitter users on Thursday will, for the first time, be able to make voice calls directly to each other through the microblogging service.
A new third-party offering from Jajah known as Jajah@call is expected to go into beta Thursday morning that will allow Twitter users to initiate a two-way voice chat with other users by typing “@call @username”–where “username” is someone’s Twitter ID–into any Twitter client. During the beta period, the company said, the calls will be limited to two minutes, but the company will evaluate that length during beta. However, it sees the two minute period–after which the call will end–as “the verbal equivalent of a tweet.”
One important element of the service is that users can keep their phone numbers private, yet be able to have voice chats with just about anyone on Twitter. To be sure, since the calls are initiated by one person, the recipient may well not be online, or may choose to ignore the call if they don’t want to talk.
There are currently several applications that allow users to create voice-to-text Twitter posts–known as tweets–but it does not appear that anyone has yet made it possible to initiate voice communications directly between Twitter users.