AT&T has started a new plan today that will allow AT&T subscribers to make unlimited voice calls to any mobile number, regardless of carrier. This is similar to Sprint's unlimited mobile-to-mobile plan. The Any Mobile Number plan will be effective this Thursday, and requires a minimum of a $20 a month unlimited messaging plan plus an existing voice plan.
This is indicative of a growing trend from carriers to stay competitive, perhaps in the wake of the Verizon iPhone. T-Mobile recently announced it is offering free phones this weekend, for example, and in addition to the new Any Mobile Number plan, AT&T has said that it would reinstate the unlimited data plan for some iPhone customers. While Sprint hasn't made significant changes to its plans aside from an increase of $10 for its smartphone plans, it is increasing its marketing efforts with phones like the Kyocera Echo (A press conference with David Blaine is quite the attention-grabber).
We won't be surprised if carriers start getting much more aggressive with their pricing strategies going forward as more phones are available on more carriers. It's a good time to start shopping around for the best deals right about now.