Google has unveiled a beta of its Google Talk instant messaging and VoIP application.

The software, however, has a major disadvantage in that it doe not offer interoperability with established leading instant messaging or VoIP vendors.

Most users of instant messaging today use networks provided by AOL, Microsoft or Yahoo! Similarly Skype dominates in the VoIP space.

The lack of interoperability is a result of other providers sticking to closed networks, complained Google on a webpage aimed at developers.

"If the people you want to talk to are all on different IM/VoIP services, you need to sign up for an account on each service and connect to each service to talk to them. We plan to partner with other willing service providers to enable federation of our services. This means that a user on one service can communicate with users on another service without needing to sign up for, or sign in with, each service," Google stated.

Google Talk at present allows users to exchange messages with third party messaging clients that use the open Jabber Extensible Messaging and Presence Protocol (XMPP).

Google Talk, however, is still in beta and the company plans to embrace the Session Initiation Protocol (SIP), a standard backed by large firms including Cisco, Nortel and Microsoft.

The VoIP service supports even fewer third party clients, offering only support to users of software from the US internet provider Earthlink and SIPphone, a startup company that is headed up by former Linspire chief executive Michael Robertson.

Google Talk is available only to users with a Gmail account. Although Google's email service is still in beta and only signs up individuals by invitation, users in the US can sign up for an account after authenticating via a SMS text message. 

The application automatically adds fellow Google Talk users from users' email service address books.

Currently the software is only available for Windows users, but support for OS X and Linux is scheduled for the future.