AT&T has dropped a major gift in to the laps of its customers today: unlimited free calling to any mobile number.

Of course, there are a few catches. The numbers do have to be based in the United States, but it doesn’t matter which wireless network you’re calling. Also, Mobile to Any Mobile will be available to “new and existing AT&T customers with a qualifying voice plan who subscribe to unlimited messaging plans.” That’s $20 per month for individual unlimited messaging and $30 per month for a FamilyTalk Plan.

All of this gets started tomorrow, February 10. It’s not going to be as big as Verizon’s big day with the iPhone 4 official launch, but it’s still something.