Following Verizon’s launch of iPhone service, AT&T has begun offering a new mobile to any mobile feature providing unlimited calling from AT&T serviced phones to mobile phones on any network, regardless of carrier.
In order for customers to access this service they must have a qualifying voice plan as well as an unlimited messaging plan ($20/month). Additional information is available at http://www.att.com/anymobile
AT&T has been increasingly competitive, recently bumping their tethering plans to offer increased data.