Out of the box, they are correct. However, the HTC 8925 is a GSM phone and ATT and T-Mobile are both GSM networks so it is technically possible to run it on T-Mobile's network. It definitely won't work on Verizon and Sprint because they use CDMA networks.
The first step is to accept the risk of unlocking the phone to work on T-Mobile's network. A failure in the unlocking process could turn you phone into a paper weight!
The next step is to go to this website:
http://forum.xda-developers.com
The forum specializes in unlocking and customizing HTC phones. Go to the Device Database and look up your phone. Typically there are several types of unlocking such as SIM unlock, network unlock, software unlock, etc. Make sure you perform all of the unlocks for your particular device.
You may want to borrow a SIM card from a T-Mobile user or buy a phone to return later to test your progress.
Good Luck!
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