Users of Microsoft's Sidekick smartphone have sued T-Mobile over the loss of personal data caused by a server failure in Microsoft's Danger subsidiary.
Two US class-action lawsuits allege T-Mobile misled customers into believing that their data was more secure than it was, according to reports.
One lawsuit claims T-Mobile's main selling point for the device was that users would always have access to their data, which would be stored safely and securely.
Last weekend T-Mobile told Sidekick users that data not currently on their devices had most likely been lost due to a server failure at Microsoft subsidiary Danger.
The other lawsuit claims that T-Mobile's advertising did not disclose that customers' stored data may not be retrieved if there was a server failure.
T-Mobile has declined to comment on the lawsuits, but said it is confident that most data will be recovered.
Microsoft said all data will be restored and only a minority of Sidekick users are still affected.
A company spokesperson said Microsoft has improved the overall stability of the service and installed a more resilient back-up process.