Ericsson, Nokia and Siemens are working together to turn GSM (global system for mobile communications) phones into walkie-talkies.

The three, traditionally rivals in the GSM network space, are jointly defining technology specifications to enable "push to talk" on GSM networks with GPRS (general packet radio service) data extensions. Initial trials of the service are set to begin the second half of this year, the companies said in a joint statement.

Push to talk will allow GSM users to push a button on their phone and talk to one or more other users in a predefined group. There is no need to enter a phone number and the sender's voice immediately sounds on the other end, much like a walkie-talkie. Operators will likely sell the service as an add-on to the standard mobile call plans.

Voice will travel as data over the GPRS network. Users will receive an IP (internet protocol) address for their phone and have to program the IP addresses of the phones they want to connect with into their handsets, according to Mats Thorén, a spokesman for Ericsson.

Like a walkie-talkie and the first internet phone applications, push to talk will be half-duplex, which means a user has to listen first and then talk.

"In that aspect, it is more limited than an ordinary mobile phone call," Thorén said.

Businesses could use the service for fleet management purposes, while consumers can stay in touch with family while on the slopes or in a theme park, according to push to talk backers.

Though vendors see uses for businesses and consumers, at least one analyst thinks push to talk might be a hard sell on consumers.

"From the corporate side, I think it is a good tool in the operator's armoury, but I am not so sure about the proposition for consumers. At the moment, the consumer can already send an assortment of different message types," said Jason Chapman, a senior analyst with Dataquest.

Mobile telephony operators will need to upgrade their networks to offer push to talk, but this is an easy and minor upgrade, according to the statement.

Ericsson, Nokia and Siemens invite mobile network operators and manufacturers to help create a single standard for push to talk on today's and tomorrow's GSM networks. The specifications that come from the joint work will be submitted to the relevant industry bodies for standardisation.