it does not mean it will work with another carrier, UNLESS the phone has the correct bands. For T-Mobile to work well, you will need the 1900 MHz band. Make sure the phone has at least that band. If it is a tri-band phone, chances are it has the 850/1800/1900 bands, and most T-Mobile tri-bands have 900/1800/1900 bands.
As far as the claim that an unlocked phone will cause constant roaming charges is pure bulls**t. Tzones may not work properly unless you have a t-mobile technician fix that for you.
Did you know that T-Mobile will unlock your phone after you have been a customer with them for 90 days?
If you want to get an unlocked phone, try to get a quad-band phone which will work anywhere in the world.
I am looking to buy a new phone off ebay. The phone i am looking at is a cingular phone, but is unlocked. I have tmobile and i heard of some rumors that using an unlocked phone will cause constant roaming charges and that the wireless content service (tzones in my case) will not work. Are these things true? are there really any downsides to an unlocked phone? I appreciate any and all quick responces as the auction is ending soon