Will a Cingular SIM work on a unlocked T-Mobile sidekick?

billy b is wrong.. the phone will work so you will be able to make calls out and recieve calls but you will not be able to use text messages or get online.

don't waste your money buying a sidekick unless you switch to tmobile.

billy b is wrong.. the phone will work so you will be able to make calls out and recieve calls but you will not be able to use text messages or get online.

don't waste your money buying a sidekick unless you switch to tmobile.

Yes, however it varies from phone to phone and carrier to carrier as to what features will work and what features will not work, I would say internet and txt/sms will not considering they probably use different systems for that but as far as calls it should bounce off the towers fine. Don't be too quick to throw the cingular phone away, like I said things may not work as well as you would like, the only way to truely find out is to try it.
I used to work for a major cell phone carrier to be un-named but it should at least make calls.

Good luck!

Let me know how it turns out.

only to make voice calls and text but t-mobile locks their browser settings

probably, though I don't know about data...

only to make voice calls and text but t-mobile locks their browser settings