jajah-2Twitter users can now make voice calls directly to each other simply by clicking on links on the microblogging service, thanks to a third-party application from Jajah.

The Jajah@call service will let Twitter users create a voice-over-Internet voice chat session by typing @call@username where the username is the callee’s Twitter identification. The calls can only last two minutes during the beta test, but that may change over time.

Mountain View, Calif.-based Jajah already operates an Internet calling service with tens of millions of users. Now this service makes it easy to make voice calls, as long as both the parties are Twitter friends and both have Jajah accounts. The service is free and should work on any Twitter-enabled device.

As with Jajah’s Internet service, you can keep your phone number private but still talk to just about anybody on Twitter. The competition is getting interesting. On Tuesday, game voice chat service Vivox announced that it could let Facebook users talk to each other simply by sharing a link. Vivox provides voice chat for 15 million users. Another new rival is TweetToCall on the Twilio service.

Jajah raised a funding round this spring. It has raised $30 million to date and has 100 employees.