NEW YORK » AT&T Inc. today said it’s adding a walkie-talkie-like application to the iPhone for its corporate customers, replicating a hallmark feature of the Nextel network, which is being shut down this summer.

A push-to-talk feature is available on some non-Nextel phones from Sprint, Verizon and AT&T, but this is the first time it’s available on the iPhone in the U.S.

With push-to-talk systems, the user pushes a button to broadcast a voice message to a group — in the case of the AT&T app, of up to 250 people. This type of service has been popular for work sites and first responders.

Sprint is shutting down the Nextel network this summer because it doesn’t support high-speed data traffic. It’s trying to get as many Nextel users as possible to switch to Sprint phones with push-to-talk capability, but it’s competing with Verizon Wireless and AT&T.

Dallas-based AT&T said the push-to-talk function won’t work just by downloading the app — the company has to work with its corporate customers to integrate it.

AT&T shares rose 40 cents to $35.85 in morning trading. Its shares have traded in a 52-week range of $32.71 to $39.