The battle between AT&T Inc. (NYSE: T) and Verizon Wireless for the title of the most "open" major carrier in the U.S. ratcheted up a few notches on the eve of the CTIA Wireless IT & Entertainment 2009 show in San Diego.

The latest move: AT&T announced that it will allow iPhone 3G users to use VoIP applications over a cellular connection. Previously, VoIP had only been allowed over WiFi connections on the 3G iPhone. (See AT&T Allows iPhone 3G VoIP and US Carriers Still Lukewarm on Skype.)

AT&T's release says Apple Inc. (Nasdaq: AAPL) and the Federal Communications Commission (FCC) were told of the decision this afternoon.

"Today’s decision was made after evaluating our customers' expectations and use of the device compared to dozens of others we offer," explained Ralph de la Vega, president and CEO of AT&T Mobility and Consumer Markets, in a statement. AT&T has other devices that allow VoIP applications on 3G, 2G, and WiFi networks.

The operator hasn't been so friendly to all VoIP applications. On September 29, AT&T filed a complaint against Google (Nasdaq: GOOG), claiming that Google Voice violated regulations. (See AT&T, Google Square Up.)

Earlier in the day, Verizon Wireless CEO Lowell McAdam said -- on the call unveiling its new deal with Google -- that it would allow the VoIP application on its network. (See Verizon & Google Build on Android.)

AT&T, meanwhile, included this statement at the end of its release announcing the VoIP changes:

AT&T allows customers to download or launch on their wireless devices a multitude of compatible applications directly from any lawful Internet website. Additionally, because AT&T uses GSM technology, the most pervasive and open wireless technology platform in the world, we support customers using any GSM phone that works on AT&T's frequencies.

— Dan Jones, Site Editor, Unstrung