Danish start-up ZYB has launched a free service, which it claimed not only backs up your phone over the air, but also makes it dynamic by tying it in with on-line social networking services such as LinkedIn, Plaxo and FaceBook.

The service, called Social Phonebook, uses the SyncML to copy your phone's contact and calendar data back to ZYB's server, where it's accessible via a web-browser.

However, product director Runar Reistrup claimed that the real value comes not from mobile backup - a very competitive market - but from the ability to also push information back to the phone from various sources out on the Internet.

When you adds a contact to your phonebook, if that person is also a ZYB user you can request their full contact details. If they agree, the data is sent over using the vCard standard for electronic business cards - and if they change their details on-line later, the changes are synced to your phone too.

Reistrup claimed that Social Phonebook doesn't create any new privacy issues, as the information it retrieves is no different from what would come up in a Google search, say. Plus, users have the option to refuse a ZYB contact request.

"I don't think people will accept just anyone on their phone. It's not for making friends, it's for enriching the information on your phone," he said. "We're not trying to change how you interact, just how you use your mobile phone."

A big challenge, he added, was creating the SyncML and GPRS settings to make all this work with hundreds of different handsets and carriers.

ZYB can currently sync a Social Phonebook account with entries on a range of business and consumer networking services. It said that it will soon add the ability to sync with Microsoft Outlook, Google Calendar and Yahoo Calendar as well.

That's likely to come in a premium version, due in Q2. Reistrup said the company plans to charge a fee for this, and also hopes to make money from adverts on its web interface.