T-Mobile USA is expanding its “Bobsled” web calling service to let anyone — whether a customer of the company or not — make free calls over the Internet from a web browser to traditional landlines and mobile phones in the U.S. and Canada.

The expanded service, announced tonight, could create new competition for Skype, which sells a similar capability based on per-minute charges or monthly subscription fees. That’s a precious revenue stream for Skype, which Microsoft is buying for $8.5 billion.

Up to this point, T-Mobile’s Bobsled has been focused on letting people call and leave messages for their Facebook friends via the web and mobile apps for iOS devices (iPhone, iPad) and Google Android.

The Bobsled web app, available here, requires a download and installation in the web browser of the user making the call.

The addition of free web calls to traditional phone numbers is a bit of a renegade approach from T-Mobile — part of the company’s “challenger” strategy vs. its larger rivals.

AT&T’s proposed $39 billion acquisition of T-Mobile is in limbo following the U.S. Justice Department’s decision to challenge the deal in court.