T-Mobile has teamed with developer Vivox to create a web-based voice chat app that runs in Facebook.

The app is called Bobsled and lets you start voice chats with your Facebook friends from within Facebook’s chat window.

We think there’s huge potential here if it catches on, and could start eating into Skype’s territory once Bobsled allows video chat and calls to landlines and mobile numbers.

In fact, the service works a lot like Skype now, but instead on relying on people to sign up for the service, all your Facebook friends are right there, ready to chat.

To use Bobsled, add the app on Facebook from the developer’s page here. You’ll be asked to install a small plugin and restart your browser.

When you log back in to Facebook, you’ll have the ability to start a voice chat with any of your friends by clicking the green phone icon next to their name.

That’s it. With one click, you’re ready to start chatting.

You can also record voice messages that are sent to your friend’s message inbox. Click the “Send Voice Message” link at the bottom of your buddy list to record one.

It’s a nice feature for people who use Facebook a lot to send instant messages.