The phones were made by Microsoft. Microsoft and T-Mobile confirmed on Monday that data including contact numbers is most likely gone following a failure of servers operated by Microsoft.
The Sidekick phones have experienced data outages for more than week.
Sidekick users are urged not to let the phone's battery run out. Any data that is now only saved to the phone will be lost if the battery dies.
T-Mobile has suspended sales of the Sidekick until the data issue is resolved.