However, to be eligible for the new plan, Unlimited Mobile to Any Mobile, customers must still purchase one of five qualifying mobile voice plans that charge for mobile calls to or from a landline phone.

Calling plans range from $39.99 a month to $109.99 a month, in addition to an unlimited messaging plan for $20 a month per individual or $30 a month per family.

AT&T's free mobile calling plan is similar to that offered by Sprint Nextel, although Sprint combines unlimited calling to any mobile phone in the U.S. with unlimited texting and unlimited data for $69.99 a month, with some added data charges for certain .

Data pricing is not affected by the new AT&T program, an AT&T spokeswoman said. Also, voice calls from a mobile phone to a landline phone or a landline to a mobile phone are still charged in the traditional way, which is why there are still five calling plans that offer a set amount of minutes for a monthly fee, she said. Any minutes over the maximum are charged 45 cents per minute.

The five qualifying voice plans that are eligible for the new Mobile to Any Mobile Unlimited program start at $39.99 for the AT&T Nation 450 plan, which offers 450 minutes of calling to or from a landline phone. With $20 per month of unlimited messaging, that plan would cost $59.99.