Go to the T-Mobile website to inquire. The Sidekick is not very comfortable to use as a voice phone. Since you cannot use the unique Sidekick features on Cingular, why try to use it for voice? You would have to get the Sidekick unlocked and keep switching
SIM cards. The Sidekick data only plan from T-Mobile cost $29.99. You would then have to get a voice plan from either T-Mobile or Cingular. That would be at least $39.99 from either carrier.