Having Windows Live Messenger on your mobile phone is a slightly strange affair. We've been so conditioned by punitive time-based data charges that the thought of leaving your phone constantly online with Messenger in the background is unnerving.

But the knowledge that Three's X-Series offers 'unlimited' browsing and messaging (although unlimited, as ever, is ultimately defined by the fair use police) eventually settles the nerves. And soon you realise the advantage that Messenger offers over standard SMS messaging.

The Messenger mobile application has a simple two-tab interface. On the one side you see a list of all your Contacts along with their current status (Online, Away, Busy, At lunch etc.).

Click on the other tab and you see a list of all the conversations you have had. Selecting a conversation expands it to so the entire list of messages is displayed.

This ends the annoying out-of-context problems that SMS messages usually suffer from, where replies are often cryptically separated from the messages they are responding to.

With Messenger all messages to and from a person are viewable as 'conversations', stored on the phone until you choose to 'End conversation'. Then the screen is cleared until someone messages you again or you intiate another conversation. Running multiple conversations simultaneously doesn't cause your brain to explode as it might in the SMS arena.

Incidentally, the alert sound that plays when someone contacts you is a lo-fi version of the standard PC Messenger alert rather than your phone's normal incoming message sound - a nice touch.

The other key advantage of Instant Messaging over SMS messaging is that you know at a glance when your mates are online and whether they are likely to respond to your messages immediately.

Certain features aren't available on the mobile Messenger. You don't have a display picture or a personal image when you are on the mobile, for instance.

There also no way to send a digital file such as a photo to a contact. Whether this is something that has been disabled at the operator's request - those MMS messages are a nice little earner - or whether it's yet to be implemented is not clear.

Group conversations are also a no-go currently, although Three's website says it will be added soon.

But the fundamental stuff such as your Buddy list and your Block list are exactly the same as they are on the PC. And you can can add to or edit them using the mobile application and see those changes applied the next time you log on via the PC.

Plus it seems a bit unfair to carp about missing items when the underlying proposition of Mobile Messenger is so compelling; the fact that you can now be a free instant message away from all your MSN and Yahoo! buddies across the planet 24 hours a day, 7 days a week (thanks to the landmark 2005 interoperability agreement between the two messaging giants).

This prospect may fill some people with dread but it's the way the world is moving. And Windows Live Messenger marks the start of a transformation in the way we communicate with our mobile phones.

In a digital age everything breaks down to data and bandwidth, whether it's voice, messaging or web browsing. And in the not-too-distant future we'll look back at bundled packages of x minutes and y text messages with bemused incomprehension.

We'll review some of the other X-Series features over the next week or two.