NEW YORK (AP) – About 4.7 million Cingular Wireless subscribers with older phones will have to pay $5 extra each month as the company tries to prod them to get new handsets so it can devote its entire network to one type of signal.

The new surcharge, unique among the major U.S. carriers, will be added to bills starting in September, the company told The Associated Press on Monday.

Cingular, jointly owned by prospective merger partners AT&T Inc. and BellSouth Corp., reported earlier this month that roughly 92 percent of its 57.3 million customers use phones based on the globally dominant technology known as GSM, or Global System for Mobile.

The rest have handsets based on one of two older technologies. One dates back two decades to the first-generation of mobile phones, which used an “analog,” or non-digital, signal to transmit calls. The second is a digital transmission technique known as TDMA, which stands for Time Division Multiple Access.

The new fee adds to a confusing array of surcharges and government taxes that, regardless of the company, can boost the average cellular bill by up to 50 percent from the advertised rate.

Like other U.S. cellular carriers, Cingular is required by the Federal Communications Commission to keep providing analog service until early 2008 so long as it still has customers with those phones.

Although the company is not required to continue providing TDMA service, it has no plans to turn off that service until the analog phaseout because both use the same portion of Cingular’s network.

Nonetheless, having to carry three different kinds of wireless signals leaves less room for Cingular to connect calls and provide data services to its audience of GSM customers.

That network capacity is crucial because Cingular is trying to shake a reputation for poor service, boasting in national ad campaigns that its customers suffer the fewest dropped calls. Cingular’s image was tarnished after its acquisition in late 2004 of AT&T Wireless Services, a merger that required the integration of two disparate networks.

In early July, a federal lawsuit was filed claiming Cingular promised to provide uninterrupted service to AT&T Wireless subscribers, but instead degraded their phone reception in an effort to persuade them to sign new contracts.

Cingular strongly refuted the claims in the lawsuit, which seeks class-action status on behalf of the more than 20 million customers AT&T Wireless had at the time of the merger. Many paid $18 “transfer” fees to switch to Cingular plans and were required to buy new phones or pay other fees, according to the complaint, filed in U.S. District Court in Seattle.

On Monday, Cingular issued a statement saying that with the number of analog and TDMA users dwindling, “the per-customer cost of using that network is increasing considerably. That’s why we made a decision to impose this charge.”

The statement stressed that, “Customers can avoid the charge by switching to our GSM network and equipment. The combination of coverage, service quality, devices, and advanced features on GSM is superior to TDMA.”