Owners of Sidekick phones might have lost all the personal information they put on the device, including contact numbers, because of a failure of servers that remotely stored the data.
The phones are made by a Microsoft subsidiary and sold by T-Mobile USA, which say many Sidekick owners' information is "almost certainly" gone.
T-Mobile is offering customers a $20 refund.
Microsoft spokeswoman Debbie Anderson said Monday there was a chance some of the lost user data could be restored from a backup system. Engineers were working at it in the Microsoft data center where the failure occurred, she said.
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The phones have been troubled by data outages for a week. Some users attempted to restart their phones by removing the battery, which erases data on the device. Normally, the data is then restored from servers, but with the server data gone, the device was left empty.
Although the underlying data services were working again Monday, T-Mobile was advising customers not to reset their phones. T-Mobile also was listing all Sidekicks as "out of stock" on its Web site Monday.
It's not clear how many customers have been affected or how many Sidekicks are in operation.