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Some users said that they would ditch their phones, while others called for a class-action lawsuit.

On Saturday, Microsoft announced that users' data stored on its servers " almost certainly has been lost as a result of a server failure at Microsoft/Danger," referring to Danger Inc., the Microsoft subsidiary that provides data services for Sidekick phones sold by T-Mobile.

T-Mobile said that it would credit users' accounts one month's worth of the data service, and offered an FAQ with tips on rebuilding their contact lists.

That didn't mollify most users posting messages on T-Mobile's support forum.

"I want to be reimbursed for my data or a free upgrade, but if they don't say anything, new iPhone here I come," said a user identified as "kangchaehee" on Sunday.

"Say goodbye to the Sidekick," added another user, "ttc7152" on the same thread, which boasted more than 3,600 messages as of mid-day today. "And Microsoft can write off any plans to even think of competing with iPhone/Android. Losing data on this scale is unprecedented -- and totally inexcusable."

Others wanted users to initiate a class-action lawsuit against T-Mobile or Microsoft, or both.

"I just spoke to a lawyer and explain[ed] the entire situation," said a user tagged as "Calsmail" last Thursday. "He informed me he would be happy to start a class-action suit against T-Mobile. He said he could not only get us out of our contracts but can more than likely get $50 per contact lost."

In a joint message from T-Mobile and Microsoft posted on the former's support forum Saturday, the two companies said that although engineers were working on the problem, "the likelihood of a successful outcome is extremely low."

The message also warned users not to reset their Sidekicks by removing the battery or letting the battery drain completely, or "any personal content that currently resides on your device will be lost."

T-Mobile and Microsoft promised to update users later today on the progress, if any, that's been made in recovering the lost data.