Twitter 'launches VoIP service'

The VoIP service lets site members make a call to their friends even if they do not know their phone number.

Developed by IP communications firm Jajah, the application limits calls to two minutes - believed to be the time needed to write a 140-character tweet on the social networking site.

Users are asked to register their telephone number and screen name with Jajah, before sending a message with the name of the person they would like to call.

The service is currently being trialled in the US and Twitter predicts it will be available more widely by Christmas.

The news follows the announcement of a similar VoIP service from Facebook called Web Voice, which allows members to make a free call to anyone in their friends list.