Not to be out done by Facebook (Berman Post: Facebook Getting Voice Chat), Twitter will be getting voice chat. The service is through a third party called Jajah. In Twitter-like form, the conversation (at least during beta) has a hard upper time limit of two minutes.
"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. "