AT&T is expanding a trial of its push-to-talk (PTT) service as it seeks to attract customers leaving Sprint’s iDEN network, which is slated for shutdown next summer.

The trial of its HSPA-based PTT service is now available to double the number of business users eligible to participate in the free program, AT&T said today.

“AT&T expects that its Enhanced PTT solution will attract many of the users being forced to migrate from iDEN,” it said.

The operator currently offers a basic walkie-talkie-style service. It is positioning its “enhanced” PTT service as an alternative to Sprint’s new CDMA-based Direct Connect offering, Sprint’s replacement for iDEN.

AT&T’s upgraded PTT service will be available on up to five smartphones when it goes live later this year, including “multiple devices” from Samsung, it said.

The service, first announced in February, is based on Kodiak Network’s InstaPoC technology. Features of the platform include simultaneous voice and data, large contact lists, faster call setup times, integration with business applications like GPS tracking, and interoperability with existing mobile radio systems such as land mobile radio and private mobile radio.

Sprint is working to move customers off its iDEN network and onto its Direct Connect service. It recently stopped selling iDEN phones on its website and is phasing the devices out of retail stores.