AT&T, which is losing its hold on the iPhone to Verizon, has introduced a new feature to its customers that offers unlimited calling to any mobile number in the United States.

AT&T's new offer is an attempt to counter an impending switch of iPhone customers to Verizon, which will be debuting iPhone 4 starting Thursday.

Verizon iPhone 4 is available for a suggested retail price of $199.99 for the 16 GB model and $299.99 for the 32 GB model with a new two-year customer agreement. Verizon customers can subscribe to a Nationwide Talk plan (beginning at $39.99 for 450 minutes) or a Nationwide Talk and Text plan (beginning at $59.99 for 450 minutes and includes unlimited text, picture and video messaging), as well as an unlimited Email and Web data plan for $29.99 per month.

Beginning Thursday, AT&T's new offer will be available to its customers with a qualifying voice plan who subscribe to unlimited messaging plans. Existing customers with an unlimited messaging plan can activate Mobile to Any Mobile by visiting www.att.com/anymobile.

According to AT&T, Mobile to any Mobile Calling minutes may be used to call any wireless number from within your calling area without using your anytime package minutes. When you're in AT&T's nationwide coverage area, you can use Mobile to any Mobile Calling minutes to place a call to another wireless subscriber in the U.S., Puerto Rico, or the U.S. Virgin Islands and receive a call from another wireless subscriber in the U.S., Puerto Rico, or the U.S. Virgin Islands.

Unlimited messaging is available for $20 per month on an individual plan and $30 per month for a FamilyTalk Plan, which allows for up to five lines, AT&T says.

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, says David Christopher, chief marketing officer, AT&T Mobility and Consumer Markets.

Sprint has rolled out a similar plan called Any Mobile, Anytime, since 2009.