AT&T, trying to maintain its visibility on the eve of the release of the Apple iPhone on Verizon’s network, today announced it was giving away unlimited mobile-to-mobile calling, regardless of what cell phone carrier the called party uses.

Beginning Thursday, Mobile to Any Mobile will be available to new and existing AT&T customers with qualifying Nation and FamilyTalk voice plans who also subscribe to unlimited messaging plans. Existing customers with an unlimited messaging plan can activate Mobile to Any Mobile by visiting www.att.com/anymobile. The URL will be available beginning Thursday.

An unlimited messaging plan costs an extra $20 per month for individuals, $30 per month on a FamilyTalk plan, covering up to five lines.

“Mobile to Any Mobile is an exciting offer that will keep our customers connected to the people they want to talk to, when they want to talk to them, without the hassle of watching minutes,” said David Christopher, chief marketing officer, AT&T Mobility and Consumer Markets.