Can I put the SIM card from an AT&T phone into a Tmobile sidekick?

[Holly]

I don't want to change plans or sign a new contract, but I do want a new phone for right now. Do I have to put the SIM card from my AT&T razor into another AT&T phone, or will it work on any phone?

[geeth]

Ya sure the AT&T sim will work in any other GSM phone only if it is already locked with AT&T or else unlocked one else if the phone is locked to some other network provider then you can't use AT&T sim in it until you unlock it .You can't use sidekick mobile with AT&T sim until you unlock it from t-mobile .You can easily unlock your sidekick phone through online providers like SimpleUnlocking.com here they are rendering network unlock code for all mobile models at reliable cost .You first verify that whether the code is available for your mobile model ,country and network provider .If yes then pay for it and unlock your phone then freely use it with any GSM sim ...