Microsoft has started beta testing its Windows Live Messenger service, which includes features such as voice over IP.
The limited public beta release is part of a move by Microsoft to revamp its existing MSN Messenger instant messaging service, which allows users to share short live messages over the internet.
Microsoft demonstrated a preview of Windows Live Messenger at the beginning of last month, with Microsoft chairman Bill Gates saying it was part of the company's strategy to offer more on-demand internet-based services.
To deliver the VoIP service to users in the beta trial, Microsoft has signed an agreement with telco MCI, which will initially allow beta testers to make free calls in the US, France, Germany, Spain and the UK.
Microsoft said the beta would be available to "tens of thousands" of testers, and would be expanded during the first half of 2006.
As well as VoIP facilities, Windows Live Messenger allows users to drag and drop files into live conversations, so users can share data and collaborate on changing it.
Companies including Yahoo and AOL are also conducting trials of VoIP services via their web portals.